Although he practices safe sex, Simon panics when his girlfriend of the moment, Sandra (guest-star Olivia Allen), confesses to him that she has an STD. Meanwhile, Martin and Mac (guest-star Kyle Searles) catch Ruthie's boyfriend, Vincent (guest-star Thomas Dekker), on a date with another girl. Also, Matt, Lucy, Kevin and Ruthie have second thoughts about telling Eric and Annie that Mary (Jessica Biel, who does not appear in the episode) is pregnant again, but also to hide the fact that she has deserted her husband Carlos and her baby son Charles. Lastly, Annie decides to go to New York to pay Mary a visit.