Hoping not to ruin their Christmas, Billie tries to keep the fact that Zack is living with her from her parents. Meanwhile, Nick makes an effort to impress his father-in-law.

When Marshall picks up smoking again, it prompts Lily, Ted and Barney, ex-smokers all, to succumb as well. Meanwhile, Robin gets a new co-anchor at her show.

During a political protest an unhappy farmer threatens the U.S. Treasury Department building by parking his tractor full of explosives nearby. Meanwhile, Loker has to bear down with a group of third graders in a field trip stuck in the Lightman Group offices.

Eric Delko returns as an expert witness for the defense, pitting himself against his old team.

Charlie helps Jake cheat on his girlfriend while she’s away for the holidays. Evelyn brings a television producer to Charlie’s Christmas dinner.

Leonard’s mother (Christine Baranski) visits, which pleases Sheldon but upsets Leonard.

Previous cases come up for review when an LAPD crime lab is discredited. One such past case involves Provenza and his ex-partner (Beau Bridges), who emerges from retirement to help revisit the investigation. The busy case reviews also threaten to undo Brenda and Fritz’s planned vacation.

The 1999 death of a high-school debate champion, which was originally ruled a suicide, is investigated after new evidence suggests he was murdered. The probe reveals the victim received a scholarship at a top private school and died shortly after losing a debate at the competitive campus.

Lenny shows up at Trainor’s after his wife goes on a trip to Mexico; Adele gets ready for Christmas with the family.

The Rapture happens on Christmas Eve, and Stan is left behind.