Chuck learns more about Sarah's past when they run into her old high school nemesis, Heather Chandler (guest star NICOLE RICHIE - "The Simple Life"). Sarah's teenage insecurities come to the surface and she tries to avoid Heather at all costs. Chuck, on the other hand, does everything he can to hear more about Sarah's hidden life. When Heather's nerdy husband, Mark (guest star BEN SAVAGE - "Boy Meets World"), turns out to be a key player in a new mission, the agents must attend Sarah's high school reunion to prevent the sale of plans for a potentially dangerous super bomber -- but the reunion ends with the ultimate cat fight. Meanwhile, Big Mike leaves town for a day and Lester -- as Buy More's new assistant manager -- decides to implement a new sales policy.