Story happens after the season finale of the originals.

Klaus and Elijah didn't die.

Because the hallow was getting more and more dangerous in Klaus's body, Klaus has the idea of killing himself in order to protect his daughter, but Caroline had a better idea and convinced her best friend Bonnie to create a new prison world, so they could put Klaus in there until they would find a way to take down the hallow and let Klaus survive.

Once it's found a way to take the hollow down without hurting Klaus, Caroline and the Mikaelsons go to the prison world to rescue him.

Now Klaus can finally be with his family and his daughter like he had been wishing to. Klaus decided to help the school as a thank you and for a way to get closer not only to his daughter but also to Caroline Forbes, who he still fancies after so many years.

(I thought about this plot while I was listening to a song called find my way back, but that doesn't need to be the title of the book.)