S5, E17: Still distraught over her breakup 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 the 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. Meanwhile, Lionel continues to get closer to Martha.