Returning early from maternity leave, Bailey (Chandra Wilson) is anxious to repay a favor to Addison (Kate Walsh) by treating her for a delicate "feminine" problem. Izzie (Katherine Heigl) grows ever fonder of heart-transplant patient Denny (Jeffrey Dean Morgan), much to the dismay of Alex (Justin Chambers). After spending the night (and morning) together, George (T.R. Knight) and Meredith (Ellen Pompeo) can't stop quarrelling at the workplace; and in a later development, George falls downstairs--and into the hands of attractive orthopedic surgeon Callie Torres (Sara Ramirez) in her first series appearance). The arrival of Mark Sloan serves only to remind Derek (Patrick Dempsey) of why he broke up with Addison. And Burke (Isaiah Washington) can't help but notice something wrong in the relationship between an aneuryism patient and his future bride. ~ Hal Erickson, Rovi