Crossover Shipgirls: Building a Dimensional Empire from Scratch

The Nasuverse World

Earth, Antarctic Continent

Atop a glacier over six thousand meters above sea level, a building resembling an astronomical observatory lay concealed.

Under the protection of powerful magic, it remained imperceptible even at close range.

Only grand magi or those specially invited could glimpse the full scope of this mysterious structure.

[Humanity Continuation Protection Organization - Chaldea].

Inside, Chaldea looked like it had been hit by a massive explosion. The place was in ruins.

A man with orange hair wearing a white lab coat stood before a glowing, blue, globe-like device.

He was speaking gravely to a young girl standing before him.

”Candidate number 48 for Mastership, Ritsuka Fujimaru.”

”I know this may feel like coercion, but I have to ask.”

”If you want to save humanity and secure a future beyond 2016, from this moment onward, you must stand alone—ready to face seven epochs of human history.”

”Are you prepared for such a responsibility?”

His tone was heavy, as if entrusting her with the fate of humanity itself.

The girl in front of him had orange hair tied into a side ponytail and orange-hued eyes. She wore a white jacket over a black skirt and stockings, exuding youthful energy.

Beside her stood another girl with short pink hair dressed in combat attire.

”… If I can do it…”

Ritsuka hesitated for only a moment before preparing to answer.

Whoosh!

Suddenly, a holographic screen materialized before the three of them.

The upper half of Setsuna’s figure was projected on it as he casually raised a hand in greeting.

”Hello? Hey? Ritsuka Fujimaru?!”

”Oh, I connected to Chaldea directly? Female Gudako, huh? Not bad luck…”

”So, how far along are you guys? What Singularities have you cleared? Any Lostbelts yet?”

The man on the other side mumbled strange things to himself.

Judging by his surroundings, he seemed to be in a conference room, with several curious shipgirl heads occasionally peeking into the frame.

”Hi~~ friends from another world, greetings~”

Yukikaze swayed her head side to side and waved.

”???”

The orange-haired girl was visibly shocked.

”W-w-w-who are you?!”

”My name is Setsuna. I’m from the Starsea Alliance, another world,” Setsuna said, keeping his introduction concise and to the point.

He glanced around at the environment on the other side of the communication.

It seemed he had directly connected to the protagonist of Fate/Grand Order.

But judging from the situation...

”Uh… has your humanity collapsed too?”

”Too?”

The three people on the other side were momentarily stunned by his choice of words.

After a brief pause, the man in the lab coat was the first to speak.

”I’m Romani Archaman, Chaldea’s chief physician. You can call me Doctor Roman.”

”You’re not wrong. This place just experienced the ’Incineration of Humanity’ event. Humanity has already perished, and only Chaldea, being independent of the world, has managed to survive—for now.”

He couldn’t ascertain Setsuna’s identity, but given his apparent ability to establish cross-dimensional communication, it was clear the state of their world couldn’t be hidden from him.

Thus, Roman decided to speak honestly.

”So, are you planning to repair the seven Singularities, defeat the mastermind behind the incineration of humanity, and save the world?”

Setsuna asked with a smile.

As a seasoned FGO player, he was all too familiar with the current stage of the story.

It was the perfect time—right after the prologue, where things were just beginning to unfold.

”???”

”How do you know?! Who even are you?!”

Both Dr. Romani and Ritsuka Fujimaru were shocked.

”Second playthrough. Ahem…”

”Anyway, let’s get to the point. I have a proposal—how about we work together?”

Setsuna coughed and adjusted his tone.

”Chaldea provides the technology and Heroic Spirits, while I supply the resources and manpower. Together, we’ll sweep through the Singularities and hang Goetia on a lamppost. How does that sound?”

”????”

Half an hour later.

Whoosh!

Through an opened dimensional portal, Setsuna and several accompanying shipgirls set foot on the soil of another world.

Blabbering nonsense through a holographic screen was one thing, but meeting face-to-face was always more effective.

”…”

Ritsuka Fujimaru, Dr. Romani, and Mash Kyrielight examined the newcomers.

On an expedition, Setsuna and the shipgirls were dressed in the standard Starsea Alliance uniforms.

The black-and-red design with the eagle-flag emblem radiated an aura of authority and gravitas, giving them the appearance of a secretive organization.

”You can cross time and space? You’re from another world?!”

Romani stared at Setsuna with a level of seriousness that far surpassed his earlier appeal to Ritsuka Fujimaru.

”Yep. Hello, Solomon—er, I mean, Dr. Roman.”

Setsuna greeted him with a handshake, his tone amicable.

Perhaps due to the abundance of supernatural phenomena in the Nasuverse, neither Romani nor Mash seemed overly surprised by the revelation that Setsuna was from another world.

Their reaction was notably calmer than the shock displayed by the Demon Slayer Corps or even the shipgirls during their first encounters.

Only Ritsuka Fujimaru seemed utterly flustered, her gaze darting back and forth between Setsuna and the shipgirls.

Buzz.

Out of habit, Richelieu and Taihou activated their scanning devices to analyze their surroundings.

Moments later, their expressions shifted dramatically.

”Commander,” Richelieu said, her voice sharp with alarm.

”Outside this facility… humans—no, this entire space-time—has already collapsed!”

If the world Setsuna hailed from was a wasteland left behind after humanity’s destruction, then this version of Earth had suffered something far worse: humanity’s very existence and history had been erased.

The fabric of the world itself had become wildly unstable, a chaotic whirlpool of distorted space-time.

Chaldea was like a space station adrift in the cosmos, existing independently from the ruined timeline around it.

”I know,” Setsuna replied with a nod.

”Both of our worlds’ human civilizations are screwed.”

”But it seems like yours still has a chance of being saved.”

...

Ten minutes later.

After a brief exchange, both sides began to gain a basic understanding of each other.

Setsuna gave a concise overview of the situation on his side, mentioning that he hailed from another world and possessed powers beyond the scope of the Nasuverse. Of course, he didn’t delve too deeply into the details about shipgirls.

The group on the other side listened with a mixture of bewilderment and intrigue.

Seeing that Setsuna had no apparent hostility, Dr. Romani began to explain Chaldea’s situation.

The Chaldea Security Organization was established through funding from various nations, with the primary goal of ensuring the long-term stability and continuity of human history.

In a sense, it was the ”center” of the Nasuverse’s human civilization.

But this world had recently suffered the catastrophic event known as the Human Order Incineration Incident.

”Someone has deliberately destroyed several critical foundation eras in human history, erasing the future of humanity after 2016,” Romani explained.

”This means that the history of this world will come to a complete halt in 2017, and human civilization will be extinguished.”

Romani gazed solemnly at the massive Earth model before him. Several glowing dots marked locations such as France and Central Asia.

”Ah? I thought humanity here got wiped out by some planet-destroying weapon.”

Formidable curiously peered at the model.

”In simple terms, the development of human history hinges on several pivotal moments,” Setsuna explained.

”For instance, the Hundred Years’ War between England and France, the dawn of the Age of Exploration, or the establishment of Mesopotamian civilization…”

”These moments serve as ’Singularities’ in history. If tampered with, they can cause severe disruptions to the subsequent timeline of humanity.”

Setsuna’s explanation likened it to a butterfly effect.

If one were to time travel and eliminate their infant self, everything related to them in the future would be altered or erased.

For human history, the death of an ordinary individual might not have much impact, but the destruction of a kingdom or the assassination of a prominent leader could drastically alter the course of future events.

Chaldea faced such a crisis.

Someone had intentionally warped seven critical events in history, creating the Seven Singularities. The cascading effect meant that human history would conclude in 2017 unless these Singularities were resolved.

”Wow, that’s so complicated… Are we here to save the world?”

The shipgirls marveled at the explanation, murmuring among themselves.

”So, can we cooperate?”

Setsuna smiled as he posed the question.

”You see, Chaldea doesn’t seem to have many people left, right? I can provide funding and manpower to help resolve the threats posed by the Singularities.”

”In return, I’d like access to this world’s magical knowledge—be it divine-era sorcery or modern magecraft. I’d also like materials related to summoning Heroic Spirits.”

While he could study it all on his own eventually, having someone else assist him was far more convenient.

Especially since he knew Romani’s true identity—the King of Magecraft Solomon.

Though currently an ordinary person, Solomon’s understanding of the Nasuverse’s magical systems was unparalleled.

”…”

Romani fell into deep thought.

This visitor from another world was undeniably strange.

Despite coming from another world, Setsuna seemed unusually familiar with the situation here.

And judging from instinct, both he and the girls by his side were no pushovers.

”You want magic-related materials?” Romani said, scrutinizing Setsuna carefully. ”Chaldea does indeed have several magi, so gathering such things wouldn’t be too difficult… provided you’re willing to help us. However, performing magecraft requires magic circuits or crests. You don’t seem to be a magus yourself, do you?”

Mash and Romani both sized up Setsuna again. He didn’t look like a magus. Instead, his demeanor felt more like that of a swordsman among Heroic Spirits.

”Don’t worry about that. I have my own methods.”

Setsuna waved his hand dismissively.

If he could digest a Wisdom Cube, there was no reason he couldn’t learn some magic.

”…”

”Alright.” After a moment of contemplation, Romani nodded.

”Getting support from another world could significantly improve our chances of saving humanity. If you can help us resolve the Singularities, I’ll arrange the corresponding materials as your reward.”

After confirming their collaboration, Romani felt a bit relieved to have new allies on board. He led the group on a brief tour of Chaldea.

The recent explosion had left the facility in shambles, with fewer than twenty surviving staff members. Most of them were busy cleaning up the aftermath.

Ritsuka was particularly fascinated by the visitors from another world and trailed closely behind Setsuna, bombarding him with questions.

”Are those girls your Servants? Can you summon Heroic Spirits too?”

”Shipgirls? What? Military ships can become Heroic Spirits?!”

”You’re going to help me save humanity? Can I visit your world someday?”

Setsuna found Gudako to be quite amusing. Compared to the shipgirls, her figure might be a bit lacking, but she was lively and had a charming, youthful appearance.

If not for his arrival, she would undoubtedly be this world’s protagonist.

But considering the current situation… why not bring her and the Heroic Spirits into his fold? In the future, Gudako could assist in commanding the Heroic Spirit forces, joining him in conquering and exploring new worlds.

”Once we’ve cleared the Singularities, you’re welcome to visit anytime.”

As they walked, an idea suddenly popped into Setsuna’s mind. He casually shared it with Richelieu:

”Since we’ve established contact with the Nasuverse, we should find a way to make good use of their resources…”

”Turning this entire world into a vassal is unrealistic… forming an alliance seems unnecessary too. Their combat strength is mostly concentrated in Heroic Spirits and a handful of magi…”

”How about we just relocate Chaldea to our world? I can have the Bulins build a new Chaldea base in Antarctica.”

”Then we could gather Holy Relics from across the world to summon Heroic Spirits, creating an elite Heroic Spirit task force… even a Heroic Spirit army sounds like it could be pretty powerful.”

From the Hyperdimensional Star Map, the Nasuverse’s strength was practically godlike compared to his own.

Its parallel worlds and Lostbelts were far too numerous.

Focusing on taking Chaldea—the heart of this world—and its Heroic Spirits seemed far more efficient.

Richelieu, already accustomed to Setsuna’s peculiar ideas, pondered briefly and nodded in agreement.

”That sounds feasible.”

”???”

Romani and the remaining members of Chaldea were utterly dumbfounded.

Vassals, alliances, Heroic Spirit combat forces—this man was saying things so outlandish they defied comprehension.

He even wanted to take the entire Chaldea with him!

”I just invited you over to help out, and you’re planning to take everything, aren’t you?!”