As you can probably discern from the title, this episode boils down to commitment. This is especially true for Isobel Stevens, also known as "Izzie," who returns to the hospital and is once again assigned to Dr. Bailey. The Nazi feels Izzie should begin anew under the tutelage of another resident, but the Chief tells her that other doctors are starting to doubt her -- essentially, Dr. Bailey's colleagues' commitment to her is waning, and she must prove herself through Izzie.
Izzie is ordered by Miranda to be seen, not heard, and to have no contact with patients whatsoever. She orders Dr. Stevens to shadow... Meredith. It's a bit of an adjustment for Izzie as she observes how many things have changed, and how little credibility she has. Against the instructions of both Miranda and Meredith, she confronts a burn patient, Gretchen, asking her point blank if she deliberately burned her hand to get out of taking the bar exam (which she failed five previous times).
It turns out that Izzie was right on the money, reaching out to Gretchen using her own experience with failure, and convincing the young woman to undergo a psychiatric evaluation. While leaving the hospital, Izzie runs into Dr. Webber, who tells her Meredith gave her an excellent evaluation. Apparently Izzie's performance is being rated on a daily basis. Richard welcomes her back to Seattle Grace.
Addison has a patient, Noelle, who has two uteruses. In each uterus is a different baby one conceived six weeks after the other. But once Addison reveals this fact, her fiance, Greg, figures out that the younger of the unborn children cannot be his. He leaves in disgust, right as Noelle goes into premature labor and requires an emergency Caesarian section.
During the operation, Addison delivers the first baby, but the second will not remain still, putting mother and child in danger. George can't keep it from squirming, but Alex saves the day by mock-commentating a boxing match (Ali vs. Frazier, 1974) to her. Earlier in the episode, Alex had taken an interest in the case and observed Greg talking sports to his little girl.
Greg eventually reconciles with Noelle. Alex cannot reconcile with Mark, however, after dropping an errand he was supposed to do for him (calling the DMV) to assist on Addison's surgery. A surprising, but genuine interest in Addison's field and the prospects of being Mark's lackey were enough to drive Alex away from a career in plastics in the end.
Also doing some soul searching thanks to Greg and Nicole is George, who is inspired by Greg's commitment to Noelle, even considering everything that took place in the last few days. He decides he wants to commit to Callie, bungling a confusing metaphor about ham and eggs and being the pig, in a way that only George can. She tells him she's tired of his indecisiveness and to quit chasing her... unless he's ready to catch her.
Cristina takes it upon herself to guard Dr. Burke's residual hand tremors by writing all his operations on the OR board. She deliberately schedules his operations at times when there will be no residents available to scrub in, and in rooms without the observation area. George and Alex feel she is abusing power, Meredith just wonders why she's so distant.
Tensions rise when she commits Preston to perform a complicated and rare surgery in which the patient's heart is removed, a tumor is scraped out, and the heart placed back in. Dr. Burke is upset Cristina went ahead and committed him to the "humpty dumpty" surgery a day before telling him, but Dr. Yang is having none of it, insisting that he will be able to pull it off. Is she out to protect him? Advance her own career? Both?
In any case, Dr. Bailey wants in on the surgery and writes her name on the OR board. Fearful Miranda might learn and expose Preston's secret, Cristina clandestinely erases Bailey's name. A devastated Bailey notices her name is off the board and believes that Burke, like some other doctors at SGH, isn't confident in her abilities. He protects Cristina and doesn't refute it, but later, Bailey sees Cristina writing Burke's surgeries on the board and realizes exactly who was behind nixing her from the operation.
Meredith questions Derek's commitment to her when he doesn't call her for a week after she broke up with McVet. So she pays an unannounced visit to his trailer. Problem is, she's the second person to do so that day. She finds him wearing only a towel, with a beautiful woman sitting on his bed. Mer leaves in a huff, learning later that it's just Derek's sister, Nancy, visiting all the way from Connecticut.
Nancy does not seem to approve of him dating Meredith and proceeds to label her a slutty intern. Visiting the hospital, she picks on Meredith, calling her names and making Derek upset. Derek is also unhappy seeing Nancy get along so well with Addison and Mark. Nancy advises him that maybe he needs a little time alone, to sort things out. To commit to himself, rather than run circles around all the women in his life.
They share a nice moment. Derek subsequently tells Meredith he does need a little time to himself, and she agrees.