The Bath Item Gift Hypothesis Christmas is a source of stress for Leonard, whose handsome colleague starts dating Penny, and his friends who are being tormented by Sheldon's obsession with gift-giving etiquette.
The Killer Robot Instability When Penny's comment about Wolowitz's love life sends him into a depressed stupor, his friends lose their most important teammate in an upcoming fighting robot competition.
The Friendship Algorithm Sheldon's intelligence, which renders him socially awkward, is also the source of his new scientific procedure for making friends.
The Financial Permeability Sheldon's "simple" solution to Penny's financial problem leads to a confrontation between Leonard and Penny's hulking ex-boyfriend, Kurt.
The Hofstadter Isotope While a fellow comic book-loving pal of the guys romances Penny, Leonard and Wolowitz venture out into the world of "Ladies' Night" at a local bar.
The Vegas Renormalization Leonard and Koothrappali take a heartbroken Wolowitz to Las Vegas, leaving Sheldon locked out of his apartment and forced to bunk with Penny.
The Classified Materials Turbulence Things begin to look up for Leonard when Penny makes a surprising revelation on her second date with Stuart. Meanwhile, Wolowitz enlists his friends' help when he discovers a critical design flaw in his project for NASA.
The Monopolar Expedition Penny is conflicted when Leonard and his friends decide to spend the summer working - at the North Pole.