Leonard saw Howard's mother out to dinner with Stuart and mentions it during lunch. Howard admits he's uncomfortable with the relationship.
Amy finds a prom dress among the clothes Penny was sorting out to make room for her new business wardrobe. It gives her and Bernadette the idea to throw the prom they never got to have on the apartment building roof. Penny's hesitant, but gives into the idea when Leonard seems excited about it.
Sheldon asks Penny about whether or not she really wants to go and he suggests that he may have sex with Amy at the end of the night since that's the norm. Penny's surprised by this.
Leonard helps Sheldon put on his bow tie. When the girls arrive Penny and Leonard share a nice moment on the roof, while Amy tries to calm down Sheldon who's alone in his room. He tells Amy she looks pretty when he comes out. He says he was nervous about having sex. She just wanted to have a nice evening and told him not to worry about that.
She starts to tell him she loves him, but he says it first. She starts freaking out and he calms her down. They head up to the party.
Howard, Bernadette, Raj and Emily share a limo to the prom and pick up Stuart and his date on their way. Stuart doesn't bring Howard's mom, but does bring Howard's second cousin who he had sex with when they were teenagers. It's awkward. After a skirmish in the limo, they arrive at the prom.
Stuart has to leave the party when Howard's mom calls.