Azkaban is a place of nightmares, and no one gets out from there unscathed, not even Sirius. It's left holes in his memory. There are things he should remember but can't, and he wants to remedy that. But there are also things he's never known and wants to find out, and he will with a little help from an old friend.

Minerva McGonagall is utterly disgusted with Albus's choice for the new Potions' professor now that the war is won, and she will make her disapproval known until she's blue in the face, but Albus has always been a stubborn old goat.