of interns, to whom they have been assigned as mentors. After failing his post-internship exam in the season three finale, George O'Malley (T. R. Knight) must repeat his internship year, to avoid being forced to leave the medical field. It is revealed that among the new interns is Lexie Grey (Chyler Leigh), Meredith's half-sister, who previously had an encounter with Derek Shepherd (Patrick Dempsey), despite his unawareness of her identity. Callie Torres (Sara Ramirez), whom Richard Webber (James Pickens, Jr.) chose for Miranda Bailey's (Chandra Wilson) righteous position of Chief Resident, finds difficulty in exposing an authoritative behavior, receiving little respect from her subordinates. It is explained that, in the aftermath of her being left by Preston Burke (Isaiah Washington) at the altar, Yang spent her honeymoon travelling with Meredith. Having just returned, she is shocked to learn that Burke has resigned from Seattle Grace, and relocated to an unknown location. She cried in response. Stevens has to deal with her romantic feelings for O'Malley, whose marriage to Torres is being threatened by his yet to be exposed affair. O'Malley, who finds himself in the unpleasant situation of repeating his internship, quickly gains support in Lexie, who decides not to tell the fellow interns about his failed exam. Karev is revealed to have visited Rebecca Pope (Elizabeth Reaser), following her departure. In the absence of Addison Montgomery (Kate Walsh), who relocated to Los Angeles, California, Mark Sloan (Eric Dane) seeks reconciliation with Shepherd, formerly his closest friend and confidant. Initially unwilling to resume his friendship with Sloan, Shepherd ultimately gives him a second chance. Bailey manifests a cold attitude towards Webber, in order to express her disappointment in not being given the position of Chief Resident, in spite of her enviable reputation among the hospital staff. Lexie meets Meredith for the first time, but is demoralized when she becomes aware of her apparent discomfort. Meredith and Shepherd encounter difficulty in being broken up, realizing that their mutual romantic feelings are an impediment in their attempt at having separate lives. The emergency room is filled with victims from a chain car accident, one of whom is admitted in the department of neurosurgery, under the care of Shepherd and Yang. As the physicians soon discover that he has been internally decapitated, they come to the conclusion that even a minor move would result in irreparable damage. A pregnant woman is admitted in