Harry and Draco get locked in a mysterious room at Hogwarts with nothing to do.

After Dumbledore's death, Harry searches for answers in the Pensieve. But something goes wrong. Trapped inside a memory, Harry finds himself back at Hogwarts in 1945 where he meets an 18 year old Tom Riddle teaching Defense Against the Dark Arts. (This story is not mine, it is by an ao3 author named paimpoint)