Sherlock uses his trademark deductive reasoning skills to find the killer of a college professor, Watson gets a call from a former lover who needs her help.

Finn thinks about his future with the glee club after sectionals are over. Elsewhere, Rachel gets ready for NYADA's winter showcase; and Kurt gets a second chance at an audition.

The new batch of interns find themselves involved in all the drama at Seattle Grace: Meredith asks Heather to go above and beyond to help fix Derek's hand; Cristina starts a healthy competition between Stephanie and Leah, which inadvertently leads to a medical emergency for Cristina's patient; Alex sees Jo in a new light as they work to save a baby's life, and April teaches Shane a valuable lesson during "lumps and bumps" duty.

After slipping into an unhealthy obsession with computer games, The Gang wrestles to define the difference between the real and the virtual worlds.

From "Hyde Park on Hudson," actor Bill Murray; musical guests Vintage Trouble

Ron's nomination for a woodworking award is threatened by the arrival of Tammy II. Jerry hosts a holiday party and some of his friends are shocked to find they were not invited.

When the machine names a New York cabdriver as a person of interest, any of his passengers could be a part of the impending crime.

A national tragedy forces Olivia to return to work at the White House. Meanwhile, we learn more about the first few days of Fitz's presidency.

When Amy gets sick, Sheldon tries to nurse her back to health; Wolowitz and his father-in-law plan a fishing trip.

Hobbit Week ends, Stephen gets the Madame Tussauds treatment, and Peter Jackson receives clarification on Tolkien lore.

Comedian and actor Eddie Izzard sings along a bit with Richie Sambora; actor Denis Leary; musician Richie Sambora as The Late Late Show House Band.

Ruxin is worried that Sofia has soured on their marriage. The guys try to shake free of an undesirable acquaintance. Jenny uses insider information to make a dirty trade.

Andre takes advantage of Kevin after his colonoscopy. Pete suspects his Grandpa Duke is committing adultery. The group enlists Rafi to settle a League dispute when no one trusts Ruxin to do it.

When the party planning committee drops the ball on the annual Christmas party, Dwight gets everyone to celebrate with a traditional Schrute German Christmas. Darryl fears that Jim has forgotten to include him in the new job in Philadelphia. Pete teaches Erin about his favorite movie, Die Hard.

When Stefan confronts Damon with a suspicion about Elena, Damon has no choice but to help Stefan investigate. Searching for answers, Damon and Stefan visit modern day New Orleans to see if they can find anyone who remembers the events of their last visit in 1942, including one of Damon’s former flames, Charlotte. Elena, Caroline and Bonnie share a girls’ night while the Salvatore brothers are out of town, and emotions run high when Elena delivers some shocking news. At Hayley’s urging, Tyler confronts one of Klaus’ hybrids, Kimberly, and the situation quickly escalates into violence. Finally, Damon is faced with a heartbreaking decision.