TONIGHT'S MUST-SEE

"CSI: Crime Scene Investigation," 8 p.m. (preceded by rerun at 7), CBS. Back when no one else seemed to care about this show, William Petersen was its most important supporter. He turned down other CBS shows and insisted on doing this one. Network officials still weren't sure; on the day before announcing the 2000-2001 schedule they reluctantly added "CSI." The result? "CSI" soon leaped to No. 1 in the Nielsen ratings. Tonight, Peter-sen temporarily exits the show he has championed, taking four weeks off to perform in a play. This episode bids Grissom farewell while giving a strong focus to Marg Helgenberger, (Catherine Willows) who is convinced that a jury has freed an innocent man and struggles to find him guilty of two other murders. In the rerun at 7 p.m., a psychic has been killed shortly after giving a dire prediction.

OF NOTE

"The Office," 7:30 p.m., NBC. When we last saw Michael, he was heading out alone on an island vacation that he had planned to share with his then-girlfriend. Now he's back and uncharacteristically content. That may not last long, especially after one of his vacation photos surfaces. Also, Pam ends up caught in an argument between Jim, whom she still likes, and his new girlfriend, Karen (played by Rashida Jones, Quincy Jones' daughter).

"Scrubs," 8 p.m., NBC. The show parodies "House" by handing Dr. Cox three medical mysteries.

"ER," 9:01 p.m., NBC. The relationship between Gates and Neela takes a hit when his former love, Meg, shows up. Meanwhile, Pratt and Hope (Mekhi Phifer and Busy Phillips) have made a disastrous mistake.