Synopsis by Hal Erickson
With the dust barely settled from the literally explosive climax of the previous episode, Meredith (Ellen Pompeo) endures the one-two punch of finding out about her mother's uniquely colorful love life. Also learning a few secrets is Burke (Isaiah Washington), who doesn't entirely like what he now knows about Cristina (Sandra Oh). Meanwhile, Alex (Justin Chambers) and Izzie (Katherine Heigl) wonder what they can do for an encore after their linen-closet rendezvous; a patient (Arlene Tur) suffering from persistent spontaneous orgasms has the staff all a-twitter; and a blast from the past drives yet another wedge between Derek (Patrick Dempsey) and Addison (Kate Walsh) with the unexpected arrival of Dr. Mark Sloan (Eric Dane) in his first series appearance). But the best is reserved for last--and it involves the hospital's Least Likely Couple.