Arthur Fleck's mum, Penny (Frances Conroy), keeps writing letters to Thomas Wayne (Brett Cullen), her former employer and father to a young Bruce Wayne (Dante Pereira-Olson), who we all know will later grow up to be Batman. Arthur opens one of the letters one day and sees that his mum is talking to Thomas about "their son, Arthur."
He confronts her about it and she admits that they were in love but had to cover up their affair. But is it legit? Can she be trusted? Not exactly — Penny, like her son, struggles with mental health issues that leave the identity of Arthur's father up for debate.