Returning for his fifth year of study at Hogwarts, Harry is stunned to find that his warnings about the return of Lord Voldemort have been ignored. Left with no choice, Harry takes matters into his own hands, training a small group of students to defend themselves against the dark arts.
When Ann, husband George, and son Georgie arrive at their holiday home they are visited by a pair of polite and seemingly pleasant young men. Armed with deceptively sweet smiles and some golf clubs, they proceed to terrorize and torture the tight-knit clan, giving them until the next day to survive.
Leonard and Sheldon publish a paper which is widely accepted except for comments from an internet troll. Meanwhile, the girls reminisce about embarassing incidents from their past.
The gang tries to help Sheldon when he becomes stuck in a rut at work by increasing his anxiety level. Howard teases Raj with a new game called "Cinnamon or Emily?"
Wolowitz tries to distract Raj, who is anxiously awaiting data from a space probe he helped launch. Meanwhile, Sheldon and Leonard go dress shopping with Penny and Amy.
Leonard and Howard turn to Raj for help in dealing with a work emergency when they nearly come to blows after blaming each other for a potential disaster at the university. Amy throws an authentic Victorian Christmas party, while Sheldon seeks revenge on her for making him celebrate the holidays.
While Sheldon says a tearful goodbye to the final installment of "Fun with Flags," Leonard, Howard and Raj search for something significant in a dead professor's research and Bernadette is shocked to learn of her co-workers’ true feelings from Penny.
When Leonard gets minor surgery on his nose, Sheldon is the one in need of sympathy. Meanwhile, Raj's parents' impending divorce causes Howard and Bernadette to work on their own marriage.
When the gang recreates a high school prom night on the roof of the guys' apartment building, Sheldon feels pressure to participate in all the typical romantic traditions.
The guys try to help Penny out of an awkward work dilemma, when a smitten doctor shows up at her door after her flirtatious sales techniques work a little too well. Meanwhile, Amy and Bernadette argue over whether or not female scientists should play up their sexuality.