Crossover Shipgirls: Building a Dimensional Empire from Scratch

”To be precise, this spacecraft consists of four spatial dimensions and two temporal dimensions.”

”The extra spatial dimension gives it the ability to traverse all barriers in the three-dimensional world. Just as the three-dimensional surpasses the two-dimensional, no technology of the three-dimensional world can stop it.”

”Including various complex force field shields or anything else—it can even freely pass through a black hole.”

Compiler spoke in a calm tone, introducing to everyone the greatness of this superdimensional spacecraft.

There are two critical milestones in space-age technology—

Surpassing light speed, and surpassing dimensions.

A faster-than-light starship possesses the ability to freely explore the universe and completely suppress ordinary starships. To a faster-than-light civilization, those below light speed are but ants.

Yet a transdimensional starship goes even further.

It breaks through the limits of the three-dimensional world. Conventional three-dimensional weapons can almost do nothing to it—just as a two-dimensional paper figure can hardly harm a three-dimensional human being.

”The Four-Dimensional Ring?”

Setsuna suddenly recalled the Ring that appeared in the later stages of The Three-Body Problem.

It was the last survivor when the four-dimensional universe of Three-Body collapsed into three dimensions—the tomb of the previous era’s four-dimensional civilization.

And it left behind a famous saying:

”When the sea dries up and the fish are gone, they will climb onto the land before it dries, fleeing from one dark forest to another.”

It was through this that the darkest secret of the Three-Body universe was revealed.

”Yes, that ring was also four-dimensional. Theoretically, its structural principle is the same as this ship... though ours has two more temporal dimensions.”

Purifier scratched her head, looking up at Setsuna with a playful expression.

”Building this thing was really hard! I’ve been working day and night, and now I’m starting to lose hair, Commander~ I demand workers’ compensation~”

”Alright, alright. Next time I’ll reward you with a one-day tour aboard the Eternal Snowfall.”

Setsuna rubbed her head.

He boarded the shuttle, preparing to embark.

A group of girls surrounded him—shipgirls, Spirits, humans, and Heroic Spirits alike, including Kanae, Fu Hua, Esdeath, Jeanne d’Arc, and Jeanne Alter.

They had seen plenty of faster-than-light ships before, but this was their first time hearing of one that transcended dimensions.

The Siren girls had been building it at the Starbase for a long time, and everyone was curious about what kind of powers it truly possessed.

Bzzz—

The spatial perception of the transdimensional starship was utterly distorted.

From afar, the vessel docked within the shipyard looked massive.

Yet as the shuttle approached, its size did not follow the rule of perspective—it remained unchanged, fixed against the starry background.

Only when they arrived beside it did its towering hull suddenly appear before their eyes, startling everyone.

”The ship’s internal structure currently exists in four dimensions. Be mentally prepared—don’t move around recklessly.”

Observer waved her tentacles as she spoke.

Click—

The hatch opened, and Setsuna was the first to step inside.

To those following behind, his body vanished instantly, prompting them to hurry after him.

”Hm? What’s that?”

Yat Sen suddenly frowned.

She looked up at the sky, feeling as though someone was watching them from the void.

She stared for a long time, but saw nothing. Even the sensors detected no abnormalities.

After a moment’s thought, she dismissed it as an illusion and followed the others into the ship.

...

”Ah...”

The instant they stepped inside, everyone froze.

The sensation of being within four-dimensional space was impossible to describe with words—it was the one thing three-dimensional beings could never truly put into language.

Just as Compiler had said, within three-dimensional space, one could see the entirety of a two-dimensional plane world.

In four-dimensional space, one could likewise see the entirety of the three-dimensional world.

From the four-dimensional perspective, nothing in three-dimensional space could hide. Every closed structure became transparent.

All concealment, all barriers—ceased to exist.

Everything was laid bare.

It was a simple rule, yet visually, it was indescribably overwhelming.

What each person faced first was information equivalent to billions of times what a three-dimensional brain could normally process. Their minds struggled to comprehend the flood of data pouring into their vision.

In Setsuna and the shipgirls’ eyes, the battleship docked beside them unfolded like a grand, living painting.

They could see all the way to the stern, and all the way to the bow—every compartment, every sealed container, all laid bare before their sight.

They could see the coolant flowing through the pipes, the faint blue glow within the dark matter reactor, the hidden missile silos deep within the hull—and every individual erosion missile within, along with every intricate internal mechanism.

Every delicate component of the axial-based weapons, every Angeloid stored in the hangars, and even every piece within each Angeloid’s body...

Everything was visible. Every single detail existed side by side, with not the slightest obstruction to their vision.

”Ah...”

Setsuna and the girls stood frozen in place for a long moment.

They were drowning in a sea of visual information, as though all the details of the universe had converged around them—vividly arranged in dazzling, endless colors.

”I feel a bit dizzy...”

Formidable shook her head. Beside her, Fu Hua stared wide-eyed, equally stunned.

Inside the transdimensional ship, they were confronted by an entirely new visual phenomenon—infinite detail.

In the three-dimensional world, human vision is limited by finite detail. No matter how complex a scene or object may be, its perceivable details remain limited.

Given enough time, one could eventually observe most of them.

But from the four-dimensional perspective, every layer of a three-dimensional object was simultaneously exposed to sight. What was once enclosed or hidden now appeared parallel and side by side.

For example, with a closed container—you could instantly see the objects within it. And those inner objects were themselves transparent, each revealing yet deeper inner layers.

Within this infinite cascade of exposure and parallel layering, infinite detail emerged.

Even the smallest thing—a cup, a pen—displayed infinite structure, and the eye received infinite information.

When every level of an object is laid bare before the four-dimensional gaze, it creates a dizzying sense of depth.

”Wait?! What is this?!”

Kanae let out a startled cry.

She, Kanao, Shinobu Kocho, and Fu Hua looked at one another—each seeing the other’s body rendered in boundless, layered detail.

Every human structure, internal and external, was exposed to sight.

Kanae looked down at Shinobu and could see her heart beating inside her chest, blood flowing between her ventricles.

Then she looked at Nyarlathotep—only to find nothing but swirling chaos.

As expected of a higher-dimensional eldritch being.

”Try not to move your hands around,” Compiler warned. ”You might accidentally touch your own—or someone else’s—internal organs. Though, given your physical constitutions, it shouldn’t be a big issue.”

”Ah...”

The brain, built for three-dimensional space, could not process the infinite information pouring in. Everyone stood dazed, paralyzed by sensory overload.

”H-How are we supposed to exist like this?!”

Formidable cried out, frozen in place—completely lost amid the ocean of infinite data. Her spatial and directional senses were utterly scrambled.

She was terrified that if she took even one step, she might accidentally walk into someone else’s body.

”Commander, maybe condense the extra dimension for now,” Observer suggested. ”We still live in a three-dimensional world. Better to stick with a three-dimensional form for daily use.”

Shhh—

Setsuna opened the holographic interface.

[Current Spatial Dimensions: Four]

[Confirm—Compress Fourth Spatial Dimension to Microscopic Scale]

Shhh—

A cascade of surreal lights flashed before everyone’s eyes, just like when the Sophon had once compressed itself from higher dimensions into three.

But the scale of this starship dwarfed the Sophon countless times over.

Bzzz—

There was a brief blur—their field of vision contracted to a single point, then expanded again.

In the blink of an eye, their sight returned to normal.

The interior was now identical to that of any ordinary starship.

The bridge, the command tower, the main control room—all shared the same layout as a standard battleship.

Their vision could no longer pierce through matter; they could only see what existed before them in the three-dimensional world, receiving finite information once again.

”Phew—”

Some exhaled in relief, while others seemed a little reluctant.

After all, compared to the three-dimensional world, the four-dimensional realm was immeasurably vast—endless within every inch.

”Commander,” Compiler explained, ”during battle, the ship can be expanded back into four dimensions. No three-dimensional defense can resist an attack from higher space.”

”Likewise, unless the enemy possesses the power of dimensional reduction, no conventional weapon can harm a four-dimensional ship.”

From a four-dimensional perspective, every part and every detail of the enemy would be completely exposed—allowing one to strike lethally from any nonexistent angle, even from within the enemy’s body itself.

”Whoa...”

The shipgirls gasped in awe once more.

”Didn’t we used to call sub-light civilizations ’natives’?” someone murmured.

”Now that a transdimensional ship exists... does that make all three-dimensional worlds the real natives?”

”...”

It took everyone a while to recover from their first experience witnessing four-dimensional space.

After carefully confirming, they found that once compressed back to three dimensions, the battleship looked far more normal.

It was indistinguishable from any other battleship—externally, its turrets, armor, and hull plating looked the same; internally, its engines and interior decor were identical as well.

The strange visions from earlier were simply the visual distortions caused by a higher-dimensional object projected into three-dimensional space.

”Let’s just keep it like this most of the time. We’re still three-dimensional beings, after all.”

Formidable rubbed her temples in relief before finally sitting down on a chair.

Earlier, her spatial sense had been so severely distorted that one wrong step could’ve landed her right inside the engine.

”You’ll get used to it after staying in four-dimensional space for a while,” Observer said, waving her tentacles lightly.

”When we first experimented in the lab, we were dizzy for quite some time. But once you fully understand the structure of four-dimensional matter, you’ll realize that working efficiency in higher dimensions is vastly greater than in three.”

”For example, when processing an object, you can skip the disassembly step entirely—just operate on it from the inside.”

”The same goes for surgery. In four-dimensional space, you don’t even need an operating table or to make any incisions—Unit 213 can perform every procedure entirely within the body.”

”...”

Akashi and the young researchers from the Life Sciences Division exchanged amazed looks. This was indeed an impressive application of dimensional technology in daily life.

”Is it difficult?”

Kayo Senju raised the most essential question.

The Siren Research Department worked semi-independently from the Empire’s Science Division, and she was deeply interested in the Sirens’ advanced black technologies.

”It’s very difficult—and very complex.”

Purifier spread her hands, answering honestly.

”After returning from the Three-Body world, it took us a long time to complete the theoretical framework for dimensional technology. As for the cost of this transdimensional ship... let’s just say it’s not much cheaper than the Eternal Snowfall.”

”If you study for a year or two, you might grasp its basic theory. But to actually work on cutting-edge research, that’ll take much longer.”

”Oh...”

The little researchers nodded earnestly, ready to dive into this new and mysterious field.

”So besides four-dimensional space, can this ship eventually gain even more spatial dimensions?” Midori Fuse asked curiously, tilting her head.

”In theory, yes. Each added dimension introduces a new spatial axis, which increases the amount of information by nearly infinity compared to the lower dimension.”

”With our current technology, constructing four spatial dimensions is already the limit. As for what five or six spatial dimensions would look like... that’s beyond our imagination for now.”

Compiler’s calm voice carried a trace of awe.

Four-dimensional space already allowed them to see everything within the three-dimensional world. Anything beyond that was almost incomprehensible.

”The Zero-Homers would probably die of shock, nanoda.”

Yukikaze twitched her ears and leaned forward mischievously.

”Who knows? Maybe we really can restore the eleven-dimensional paradise era someday.”

In the Three-Body world, the dimensional war had dropped reality from eleven dimensions down to three—and was close to collapsing into two.

The Zero-Homers and several god-level civilizations sought to revert the universe back to zero dimensions and restart it, hoping to return to that primeval paradise.

In contrast, the Starsea Empire’s technology advanced in the opposite direction—ascending from three to four dimensions and beyond.

”Raising one’s own dimensional level and altering the universe’s dimensional level are two entirely different things,” Compiler said, shaking her head as she turned toward Setsuna.

”The four-dimensional explanation is over, Commander. Don’t forget—this ship is six-dimensional.”

”In addition to four spatial dimensions, it has two temporal ones.”

”Oh.”

Setsuna perked up, interest flashing in his eyes.

”It can travel through time, right?”

”Yes.”

The Siren girls led him into the main control room, where a massive holographic display unfolded before his eyes.

Setsuna was already intimately familiar with the control systems of ordinary starships, but this transdimensional vessel had some subtle differences.

”For this ship, time isn’t linear. It can freely traverse both the past and the future.”

”Everyone should already be familiar with the concept of going back to the past, right? The Spiritron Transfer Device can do something similar, but the transdimensional ship makes it much easier—no preparation required.”

As she spoke, the Siren girl guided Setsuna to press a button. A new prompt appeared before him:

[Active Reversion: Please Select Target Time]

”Hm?”

”Active reversion? There’s a passive one too?”

Setsuna raised an eyebrow.

”We designed two types of temporal reversion,” Compiler explained calmly. ”Active reversion allows you to choose when to travel into the past.”

”Passive reversion activates automatically when the ship suffers critical or fatal damage—it resets itself to maintain a perfectly intact state.”

”Oh.”

Setsuna nodded in understanding.

That feature’s practical value was beyond question. Under attack, the ship could reset its own timeline, nullifying all damage. A truly formidable ability.

He glanced at the interface and selected a target time—half an hour earlier.

Shhh—

The scene before them flickered. The ship seemed to enter a temporal corridor, much like when they traveled between worlds.

Their perception stretched, then compressed again.

In the next instant, the starship emerged into a strange void.

”Hey— isn’t that us?!”

Formidable pointed downward.

Below them was the familiar Starbase.

A small shuttle was carrying Setsuna and the shipgirls toward the transdimensional vessel.

Click—

The hatch opened, and Setsuna stepped inside first, followed by the girls.

It was exactly the same scene—from half an hour ago.

”Ah...”

Seeing their past selves, everyone’s expressions turned a bit awkward.

”Hi!!!”

Yukikaze knocked on the viewport glass, waving cheerfully.

”Hm? What’s that?”

Down below, Yat Sen looked up toward the ship in confusion.

Their gazes briefly met. Sensing nothing unusual, she simply shook her head and entered the ship.

”Ah?!”

Yat Sen aboard the ship froze.

”So what I just saw earlier... was you?!”

She realized that time had formed a closed loop—a miniature paradox, like something straight out of Interstellar.

”Don’t move recklessly,” Observer warned. ”Interfering with the past could trigger a butterfly effect and disrupt the timeline.”

”We’re currently in the temporal rift—not fully manifested in the past world yet.”

”For safety, we’ll just observe.”

”Got it.”

Setsuna nodded.

Compared to the Spiritron Transfer Device, the transdimensional ship was infinitely more convenient.

This was a monumental leap in temporal technology.

That old time machine they’d stolen from Chaldea years ago—one that required manual assembly, a pile of Wisdom Cubes, and a small mountain of fuel—had finally completed its historic mission.

From now on, aside from saving humanity in Chaldea’s own timeline, worldline alterations across other universes could rely on this far more advanced transdimensional method.

”Eh, and here we were planning to burn a few stars just to travel fifty thousand years back... guess that’s unnecessary now. Plans never keep up with progress~” Grey sighed softly.

Shhh—

Setsuna canceled the reversion. After a brief hum, everyone returned to their original timeline—back to the shipyard at the Starbase.

”...”

The shipgirls exchanged bewildered looks. The four-dimensional space had already twisted their sense of direction; now, time travel had completely scrambled their sense of continuity.

It took a while for their minds to stabilize.

”What about traveling to the future then? I want to see what the Starsea Empire becomes.”

Setsuna brought up the holographic interface and selected the corresponding option.

BOOM!!!—

The starship suddenly shuddered violently.

A low hum reverberated as the view outside the portholes turned to chaos.

Countless overlapping visions interwove in the void.

If one looked closely, faint outlines of the Starbase and ships cruising through the stars could still be discerned—but everything overlapped, unstable, and incoherent.

”???”

Setsuna frowned, puzzled.

”Commander, it’s impossible to travel into this universe’s future,” came a soft, ethereal voice.

He turned around to see Dreamweaver holding a stuffed doll, quietly watching.

”Your very existence is special to this universe,” she explained gently. ”Everything you’ve brought from the other universes—people, technology, artifacts—affects this worldline.”

”The intertwining of different worlds’ causes and effects has created countless possible futures. The transdimensional ship cannot pinpoint a single definitive one.”

”I see...”

Setsuna nodded thoughtfully after a moment’s silence.

In simpler terms, the worldlines of this universe were too entangled—far beyond even a six-dimensional ship’s capacity to process.

”I really wanted to see the Empire’s future...”

”Well, maybe it’s better this way,” Gudako said with a grin. ”If we could completely predict the future, life would be boring.”

”...”

”So, if it’s impossible to travel into the future of this universe... we can still do it in single-worldline universes, right?”

Setsuna quickly realized the key point.

”Yes,” Dreamweaver nodded. ”In other universes, the transdimensional ship will work perfectly.”

”Excellent.”

Setsuna snapped his fingers and turned toward Fu Hua.

”Hua, let’s head to the future—and find a few more waifus while we’re at it.”

—

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