THICKER THAN WATER - After Max (Jason Behr) is injured in a freak car accident, Liz (Shiri Appleby) once again puts Alex's (Colin Hanks) friendship to the test when she asks him to help protect Max from medical procedures which could expose his secret identity. Meanwhile, Maria's (Majandra Delfino) cat-and-mouse game with Topolsky (guest star Julie Benz, "Buffy the Vampire Slayer") reveals the startling truth about the nature of her job and leads Alex to a bitter confrontation with Liz. Katherine Heigl, Brendan Fehr, Nick Wechsler and William Sadler also star. David Nutter directed the episode written by Barry Pullman