On Valentine's Day, everyone is preparing for the Valentine's singles benefit dance. Luke has a new crush: that being Marissa's mother Julie. Meanwhile, Sandy gives Seth some sex advice over his dating of Summer. Theresa Diaz, an old friend Ryan's, shows up in town as part of the catering services for the benefit, which forces Marissa to lay her feelings on the table that her breakup with Ryan may have been a mistake. Also, Seth and Summer take their friendship to another level.