 Michael Rosenbaum portrays Lex Luthor , the son of billionaire Lionel Luthor , who is sent to Smallville to run the local fertilizer plant . After Clark saves his life in the pilot episode , the two become quick friends . Over the course of seven seasons — beginning on the day Clark rescues him from drowning — Lex tries to uncover the secrets that Clark keeps . Lex 's curiosity eventually leads to a blowout between him and Clark in the season three finale . Lex 's investigations ultimately lead to him discovering the truth about Clark 's alien heritage in the season seven finale . 
