Simon has always been careful about practicing safe sex, so he panics when his girlfriend admits to having an STD the morning after they've slept together. Meanwhile, Martin and Mac catch Ruthie's boyfriend with another girl, and they don't know how to break the news to her. Matt, Simon, Lucy, and Ruthie know a dark secret about Mary, and they agonize over how to tell their parents, who falsely believe that Mary is pregnant again.