Thirty year old Kakashi was supposed to have been killed by Pein during the Invasion. Instead, he wakes up in the body of his twenty year old self. (It gets a lot more complicated.)

Genma is a very good assassin. He takes a job, takes a life, comes home, repeats. Then one day a stray genin shows up on his doorstep. And then another. Lonely assassin? Not so much. Try resident shinobi den mother. [In which Genma is a (very manly) mother hen, his apartment attracts strays, and all of his (bastard) friends are quite amused.]

No one in their right minds would ever expect it of him, and that’s why he’s the one best-suited to the job.

In a world where Fugaku has three sons, he wishes he'd stopped at the one. There's nothing wrong with the youngest, it's just... The middle child.