Sheldon pushes himself to the limit when he collaborates on three projects on the same day; Bernadette nears the end of her maternity leave.

Reid and Prentiss are taken hostage when they go undercover to investigate possible child abuse at an underground cult.

Jake has a windfall when Charlie takes him to the track; Alan finds himself stuck at the Department of Motor Vehicles.

When Charlie finds out that Mia is back in town, he begins to question his relationship with Chelsea; Judith is rushed to the hospital.

After saying goodbye to the late Charlie Harper, Alan meets Walden Schmidt (Ashton Kutcher), a broken-hearted billionaire.

Larry angers a hot-yoga teacher and has an auspicious first date.

The team tracks a suspect who is abducting young mothers; Beth makes a discovery about Mick.

Hal invites his friend Larry over while Lois is out of town.

Bob reconsiders his future at MaxDot and seeks advice from an unexpected source, while Abishola questions Bob's erratic behaviour.

Susan learns that Mike had been keeping a secret; Bree becomes the prime suspect in the murder of Gaby's stepfather; Gaby discovers a way to earn a higher commission at work; Tom's girlfriend works to make sure his divorce goes through.

Walden juggles his two identities; Alan ends up alone on Christmas.

Alan invests Walden's money in Kate's fashion line; leading a double life takes its toll on Walden.

When Mike reaches a weight-loss goal -- his personal best -- he decides to celebrate by indulging in a slice of pie.

Carl and Mike try to reconcile their partnership with couples counseling; after Peggy decides she won't participate in a promotional radio interview, Molly does it on her own.

Voight teams up with an unlikely ally to take a killer off the streets while fighting to bring his Intelligence unit back together.

Voight and the team work together to uncover the truth of the Bell case before more lives are lost.

Atwater reunites with an old friend from his academy days after a bombing rocks downtown Chicago.

Fran becomes a burden to her dating friends when her therapist convinces her to stop dating.

Nora is irritated by unexpected visitors; Kevin's predicament becomes impossible to ignore.

Nora travels to St. Louis in her role as an investigator for the Department of Sudden Departure.