When Penny asks the guys to her party, which will include dancing, they seriously think about not attending as they aren't "dance" people. But when they learn it's a costume party (the fact of it being Halloween irrelevant), they change their collective minds and decide to go. Leonard wants this party to be an opportunity for Penny to see him as someone in her social circle. But Leonard's goals for the evening change when Kurt, Penny's ex-boyfriend, shows up, he who seems to be an invited guest and who Penny seems happy to see. Leonard, still smarting from his first and only other previous encounter with Kurt, tries to be Penny's protector while protecting himself from a potentially painful beating from Kurt.