The Empire is a powerful military nation on a continent that looks much like early 20th century Europe. But in this world, magic is real. The Empire is involved in a war against the Entente Alliance, and fighting a poorly executed delaying action in the northern theater. Second Lieutenant Tanya Degurechaff leads a small platoon of mages. But there's something unusual about this lieutenant... she's only nine years old!

The secret of Tanya's birth is revealed. Tanya was a cold and unemotional Japanese salaryman who was pushed in front of a train by a man he laid off. An infuriated godlike creature named "Being X" sent him to this world, and reincarnated him as a baby girl... with no promise that he would ever be reincarnated again. Now Tanya has to survive, because this is all she's got!

Tanya's new battalion is off the ground and running, and it's time for them to do their first mission: fight back a massive invasion by the Dakian Dukedom to the southeast. The enemy army numbers in the hundreds of thousands, but they've brought no air units at all, which makes them easy pickings for Tanya's highly trained force. After wiping out the enemy army, she heads straight for the enemy capital and launches an attack there.

After her easy victory over Dakia, it's time for a more difficult mission: aiding the destruction of the Entente Alliance to the north. The Alliance is being backed by the strong powers, and has equipment and training that in some cases rival the Empire. When Tanya arrives, the Alliance is striking a critical supply base with a bombing raid, and the local Empire force are too overwhlemed to defend against it. Can she get there to hold them off in time?

The war with the Entente Alliance is done, and it's back to the Rhine for Tanya and her forces. The Republic troops are highly trained, well-equipped, and stronger than any she's faced before. And in a distant hospital, a former Kingdom officer wakes up, with a mission from God imprinted on his mind...

Tanya's spectacular mission is a success, and the Republic is surrounded and crippled. But on her way back, she runs into a group of allied mages, lead by Anton Souix, the man she crippled once more. He's back, and powered by Being X, and his one mission in life is to take down the Devil of the Rhine!