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!

After her stunning success with her first mission, Tanya is assigned to test a new piece of technology: a highly unstable prototype calculation jewel. This jewel offers her unparalleled power, but just using it risks her life. Tanya is determined to get out of this project and on to someplace easier and safer (like the front lines of the war), but Being X has other ideas...

Tanya is now living life on easy street, as a highly-respected student at the military university. She's so highly respected, in fact, that one of the Empire's chief military planners asks her for her opinion on the course of the war. Eager to show off her intellect (and hopefully land herself a cushy job in the planning department), Tanya comes up with a plan of astonishing insight and intelligence. Her idea backfires, however, when she turns out to be a little too smart for her own good...

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 Empire is launching a seemingly foolish attack on the Entente Alliance in the middle of winter. They lack the supplies to successfully invade. Tanya realizes that this attack is a feint intended to distract from a fjord landing mission. Sure enough, she is sent to lead the invasion of the fjords, and silence 20 cannon emplacements in time for the fleet to land and the Imperial forces to disembark.