Leonard comforts Penny after she breaks up with her boyfriend, again. After asking her out on a date they each turn to Sheldon for advice. Also, Howard teaches Sheldon Mandarin so he can complain about his not-so-tangerine-Tangerine Chicken.

After finding out Leonard has never had a birthday party Penny decides to throw one for him (and forces Sheldon to join her). Unfortunately things do not go as planned and Wolowitz ends up in the emergency room.

When Sheldon's attractive twin sister Missy comes to visit the guys fight over her. Sheldon ends up trying to be protective of her, much to her displeasure.

Leonard buys a replica time machine online. As the boys move it into the apartment they accidentally make Penny late for work causing her to lash out at Leonard for his nerdy ways. Upset, Leonard decides to get rid of all his 'nerdmabelia' and the guys tr

The guys decided to compete in a physics bowl but find Sheldon's showboating too much and kick him off the team. Upset, Sheldon forms his own team to compete against them.

Sheldon and Leonard look for a way to derail a 15-year-old physics prodigy's research.

When Sheldon gets sick, Leonard and the boys know what's in store and avoid him like the plague, leaving an unsuspecting Penny to nurse the world's worst patient back to health.

After hearing Penny sing, Leonard lies to avoid seeing her perform, but Sheldon takes it one step further with a more elaborate lie that leads to the appearance of his non-existent cousin.

When Sheldon and Leonard are invited to present their joint findings at a physics conference, Sheldon refuses. But when Penny steps in to mediate, it only deepens the rift between the two best friends.

Koothrappali's parents want him to go out with another Indian woman with the hopes of initiating a pre-arranged marriage. But his date turns out to have more in common with Sheldon.

When Wolowitz hooks up with Penny's old friend, it ruins "Halo Night" for the rest of the guys.

The guys are invited to Penny's Halloween costume party. During the party, Leonard has a run-in with Penny's ex-boyfriend.

Leonard and his colleague Leslie spend the night together. But Leslie tells Leonard after the fact that she's only interested in a one-night stand.

Sheldon's mother visits to get him out of the funk he's in after he is fired for disrespecting his new boss.

Leonard gets upset when he discovers that Penny is seeing a new guy, so he tries to trick her into going on a "date" with him.

Penny is furious and more than a little freaked out with Leonard and Sheldon when they sneak into her apartment overnight, and clean it while she is sleeping.

A pair of brilliant theoretical physicists meet a woman who shows them how little they know about the real world.