Episode: 6×19 (Season 6 Episode 19)
Name: Nemesis
Originally Aired: Apr. 26, 2007

Plot: When Lex is trapped deep inside underground tunnels by the wife of a former test subject, Clark sets off to find him but is rendered powerless by the kryptonite-laden walls. The only way the two of them will make it out alive is if they put their differences aside and help each other.