What to Watch on Friday: Mr. Monk gets his badge

Law & Order (8pm, NBC) - An elderly man is forced to defend his store from an armed robber, but Lupo and Bernard suspect an ulterior motive.

Ghost Whisperer (8pm, CBS) - Melinda worries about Aiden when he's befriended by the ghost of a little girl who died with leukemia.

Monk (9pm, USA) - Monk is finally reinstated to the San Francisco police force, and his first case, tracking down a serial killer, is more challenging than he anticipated. We see the return of Trudy tonight (Brooke Adams) and the show has promised us resolution on her murder, so expect developments on that. After this, only two episodes left. 

20/20 (10pm, ABC) - Another day, another Sarah Palin interview.