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. ~ Hal Erickson, All Movie Guide