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House
8/7c Fox
It's time to wrap another season, and on medical shows that can often mean a disaster with which the docs must deal. Tonight's calamity: A crane has collapsed onto a big building in downtown Trenton, N.J. House, who's preoccupied with the news that Cuddy and Lucas are moving in together (he even gives her a "Housewarming gift"), is most interested in saving the guilt-ridden crane operator, but a trapped woman (China Jesusita Shavers) who could lose a leg literally commands his attention. He'll show a side of himself you don't see very often. — Paul Droesch

Dancing with the Stars
8/7c ABC
We've reached the semifinals, which means that Nicole Scherzinger, Evan Lysacek, Chad Ochocinco and Erin Andrews will be battling it out for the three spots available in next week's finals. The pressure's on as the celebs continue to perform two dances each. Will the tension cause Nicole to cry once again during rehearsals? Will Evan be able to overcome the coldness Anna warned him about? And will Chad and Erin continue to improve, or will they regress at this crucial stage in the competition? — Jennifer Sankowski

Gossip Girl
9/8c CW
As the series' third season comes to a close, Blair has a big decision to make: Will she face her fears about loving Chuck and meet him at the top of the Empire State Building by 7:01 pm? Meanwhile, all is not well for Nate and Serena, as their relationship is rattled by a post on Gossip Girl about Dan. To further complicate things, Georgina comes back to town (which always means trouble); and Jenny continues her downward spiral, finally hitting bottom. — Jennifer Sankowski

Two and a Half Men
9/8c CBS
While Alan and Jake are away, the apparitions of Charlie's former girlfriends visit him, which, considering Charlie's conquests, should leave the "other" world as empty as Charlie's scotch bottle. Among the actresses making return appearances are Katherine LaNasa, Ming Na, Emmanuelle Vaugier, Rachel Cannon, Aja Evans, Jodi Lyn O'Keefe and Rena Sofer. ZZ Top make a cameo appearance. — Bill Ecklund

The Big Bang Theory
9:31/8:31c CBS
"Math, science, history, unraveling the mysteries, that all started with the big bang!" Everything starts somewhere, especially great friendships. Tonight, Leonard has a story to tell Penny about how he met his roommate Sheldon. And viewers finally find out why that elevator is always out of order. — Bill Ecklund

Castle
10/9c ABC
It's been so much fun watching the delightfulness that is Castle that it's hard to believe it's already time for the show's second season to end. Having come into its own with meatier plots, more rounded supporting characters, humor out the wazoo and satisfying heat between the leads, the show wraps things up with the case of a covert operative who appears to have been assassinated. Murder aside, we're watching to see if Castle and Beckett admit to their mutual attraction or if Demming will be around for Season 3. Avid readers should be able to spot a few notable cameos: Stephen J. Cannell, Michael Connelly and James Patterson appear as themselves. — Rhoda Charles

MLB Baseball
7/6c ESPN
Settle in for the first of the season's nine Bronx encounters for the Boston Red Sox, who've already dropped four of six Fenway Park games to defending champ New York in baseball's mega-rivalry. While Kevin Youkilis and Robinson Cano swung hot bats in the early meetings, a time-of-game side story developed when umpire Joe West couldn't help but bark about the teams' turtlelike pace of play. Tonight may not be the best game to monitor in that regard as noted slowpoke Daisuke Matsuzaka is expected to start opposite fellow righty Phil Hughes. — Roger Leister