Mensa-fied best friends and roommates Leonard and Sheldon, physicists who work at the California Institute of Technology, may be able to tell everybody more than they want to know about quantum physics, but getting through most basic social situations, especially ones involving women, totally baffles them. How lucky, then, that babe-alicious waitress/aspiring actress Penny moves in next door. Frequently seen hanging out with Leonard and Sheldon are friends and fellow Caltech scientists Wolowitz and Koothrappali. Will worlds collide? Does Einstein theorize in the woods?
Penny has a problem dealing with Leonard's confession about having kissed another woman, while Sheldon is confused about Amy calling a time out. S9, Ep1
To ease Penny's mind, Leonard pays a visit to the woman he kissed during his expedition to the North Sea. Sheldon's breakup leads him to retaliate. S9, Ep2
The guys surprise Leonard with a belated bachelor-party weekend in Mexico, but their plans hit a bump in the road when they get a flat tire. S9, Ep3
Sheldon reverts to 2003 when he learns that Leonard and Penny are moving in together, while Howard and Raj form a band to play at the comic store. S9, Ep4
When Barry Kripke gives the guys fencing lessons, he makes a point of letting Sheldon know he's interested in dating Amy. S9, Ep5
A helium shortage leads Sheldon and Leonard to do business with a dealer in the black market to acquire some of the element for an experiment. S9, Ep6
Sheldon's interview for a 'Star Trek' documentary focusing on Spock turns up a shocking revelation about his breakup with Amy. S9, Ep7
When Leonard, Penny and Bernadette learn that Amy is going out on a dinner date, the trio set out to spy on her and the guy. S9, Ep8
Sheldon and Amy spend Thanksgiving at the aquarium, Wolowitz volunteers at a soup kitchen, and Penny realises that Leonard knows a lot about her. S9, Ep9
God only knows what song is constantly playing in Sheldon's head, but it reminds him of Amy and leads to a surprising revelation. S9, Ep10
Professor Proton visits Sheldon in a dream and gives him Obi-Wan-like advice about whether to go see the new Star Wars film or spend time with Amy. S9, Ep11
The Sales Call Sublimation. Penny entices Leonard to meet with a psychiatrist, Stuart moves out of the Wolowitz house, and Howard and Bernadette find themselves at a loss. S9, Ep12
Dealing with an under-the-weather Sheldon is draining for Penny, Leonard and the gang so they decide to treat themselves to a Sheldon-less weekend. S9, Ep13
Sheldon's Meemaw visits and there's friction between her and Amy, which intensifies after the elderly woman blurts out a secret about Sheldon. S9, Ep14
Sheldon and Amy celebrate Valentine's Day, Leonard and Penny feel the pangs of aging, and Howard and Bernadette find a surprise in their hot tub. S9, Ep15
The gang learns Bernadette is pregnant and celebrates with karaoke. Meanwhile, Howard freaks out, convinced he must make more money for the baby. S9, Ep16
Sheldon and the gang celebrate his birthday and he's surprised by a special guest. S9, Ep17
Howard reconsiders entering into a legal partnership with Leonard and Sheldon, and the women offer Raj dating advice. S9, Ep18
Sheldon shocks Amy with a startling revelation after she buys him a laptop. Raj throws Leonard and Howard under the bus when they lie to their wives. S9, Ep19
Sheldon shares a secret about Leonard when the two go on a weekend trip with Penny and Amy; Raj goes all out with pregnancy gifts. S9, Ep20
During a group get-together, a minor argument between Sheldon and Leonard turns into a heated debate, leaving the rest of the gang to choose sides. S9, Ep21
Sheldon stays with Bernadette while the rest of the gang attends an event where they run into Penny's former boyfriend and meet Raj's girlfriend. S9, Ep22
Sheldon hires Stuart to spend the day with Amy. Meanwhile Leonard's mother visits, and Penny just can't seem to connect with her. S9, Ep23
Leonard's recently divorced parents roll into town, as does Sheldon's holy-roller mother, and all hell breaks loose. S9, Ep24