A young boy must pose as his ace pilot twin sister for public events because of her crippling social awkwardness. Having found his talents in softer skills as opposed to his sister finding hers in combat and engineering, they both make up a legendary propaganda idol in their world that boosts morale up enough for them to not only survive but also steadily triumph in their war against a mysterious alien race with strange magical powers.
In the final rush, they drive their forces to victory but just when they clinch it the entire battlezone gets swallowed up in a blinding explosion of light. When the boy wakes up he finds himself waking up in a recovery pod to the relief of his sister's commanding officer. He explains that it seems that their ships were sent to a fantasy world where the biggest threat to their survival aren't the aliens they've been fending off for so long but the giant beasts that populate the place.
Their mechs do well enough to fend the beasts off but their ships isolated away from each other and are grounded due to the strange properties of this world. Comms are spotty too so they only intermittent messages between the known surviving forces but what is clear is that they need to gather up at the main warship before their resources run dry.
The boy asks how long before they join up with the ship his sister was resupplying at but gets told that his sister isn't with them. They found her unit was missing from its bay and that it seemed to force its way out of the ship. They don't know why she did it but the boy has to keep posing as her until they find out.
Their ship still has her secondary unit so it becomes imperative that he learns to handle it well enough to pass off as her "recuperating" from memory trauma. While training in its simulator one day though, he receives a coded transmission from her main unit giving him its location.