The basic premise is that the MC one day found himself in a game he was playing about anime girls with powers helping fight off a genocidal alien invasion. Only problem is that he's not an anime girl, he's a civilian with nothing to his name. So, to not starve, he joins the war effort as a soldier. He meets the titular mentor, helps her become strong, which ends up getting her killed, and decides to take her place in the plot out of guilt. Eventually, thanks to his efforts, humanity wins the war. The story begins a few years after the war ended.

It should be called "ptsd the light novel" because holy sh*t everyone is f*cked.

All of the cast are either the anime girls that helped stop the war or various soldiers that participated, and half of them are actively suicidal from the war. Like this isn't a happy story, these are traumatized veterans trying to deal with a post genicidal war world.

I cannot stress this enough, these are soldiers who watched everyone they cared for die, and don't know how to deal with that because no one has therapy, because the war stretched humanity too thin for that luxury.

the MC himself contracted a terminal injury during the war and spends the beginning of the story trying to find a place to die so that none of his fellow veterans mourn him, further adding to their trauma.