S5, E17: Still distraught over her break-up with Clark, Lana experiments with a dangerous serum that allows her to flatline and see her deceased parents again. A worried Clark rushes to save Lana from hurting herself but ends up being injected with kryptonite-laced serum by an arrogant medical student. When Clark flatlines, he is able to talk to his dead father, who warns him that Lionel knows his secret.