Smallville - Season 4, Episode 19
Kevin Grady (JONATHAN BENNETT) possesses the ability to make people lose their memories. Kevin unleashes his power on Clark, causing him to suffer total amnesia. Chloe discovers Clark's malady and teaches him about his superpowers -- while simultaneously trying to keep Clark from accidentally revealing his powers to others. Consequently, Clark meets Lana, becomes enamored of her and decides to share his secret with her. Seizing the perfect opportunity, Lex tries to convince Clark to show him where the three stones are hidden.