We follow the course of a girl's life, since birth, passing through Halloween, playing with dolls, first dance, till she grows up to a beautiful young blonde in a nightclub. She's having fun, dancing and drinking with a friend. A car crashes, and the blonde girl is taken to the hospital, severely wounded and disfigured. Her survival chances are good – until someone kills her.
Arriving at the hospital, Mac and Stella learn from Flack the girl's name was Nicole Garner. The body shows signs of suffocation. No one of the nurses saw anything. She had been in a coma for five days after her accident, and had just woke up. As a result of the accident, she'd be paralyzed from the waist down. Stella remarks she was given a second chance. "Question is", says Mac, "who didn't think she deserved it?"
The team start rounding up evidence. Talking to Nicole's mother in the hospital hallway, Mac meets Julie Rollins, who's a volunteer at the hospital – and whose daughter was also in the crash but didn't survive. The conversation is interrupted by the abrupt arrival of Nicole's father, Frank Russo, demanding to see his daughter. Police officers leaded by Flack and Mac stop him from entering the room; Stella comforts him.
The hospital's head of staff, Dr. Richards, greets Hawkes in the hallway, and we learn the ME-turned-CSI once used to work in that hospital. Their conversation quickly goes from cordial to heated, with Hawkes demanding access to the staff and the doctor trying to defend his co-workers. Hawkes brings up the possibility of a mercy killing, the doctor is outraged.
Knowing the doctor's lounge had been broken into, Danny and Lindsay go process it; they find tool marks and fingerprints.
In the morgue, Sid confirms Nicole was asphyxiated; with her wounds, he states, with a pitying look, that she was "a sitting duck". There were also traces of an unknown substance on the girls lip.
Sid takes the opportunity to invite Mac to come spend Thanksgiving with his family, clearly not in the mood to let his friend say no, and Mac surprises him with the news that he already has plans. Typically, Mac doesn't say anything else, despite Sid's obvious curiosity.
The trace on Nicole's lip turns out to be krill, a tiny shrimp-like crustacean that, as Adam tells Hawkes, is a main ingredient in saltwater fish food. That leads Hawkes and Flack to interrogate Kevin Green, the maintenance worker responsible for the fish tank in the hospital's visitor lounge. The man eventually admits to entering the room and touching Nicole, but nothing else.
The chief of staff comes to the precinct looking for Hawkes, furious because Green was interrogated. In the argument that follows, Dr. Richards reminds Hawkes how hard it is to be a doctor. "This was not a mistake", says Hawkes, "it was murder." To what, Hawkes' old boss retorts: "Nobody ever accused you of murder?"
The conversation with Dr. Richards brings back old ghosts, and Hawkes sits alone in the locker room contemplating memories of his work in the hospital. His lonely sulking is interrupted by Danny, who comes in complaining he just spilled his coffee all over his shirt. While he changes, Danny notices the look on Hawkes face and tries to get his friend to talk; Hawkes tells him about his decision to leave the hospital after losing one patient too many and work in the ME's office. "You're still a doctor, Sheldon", Danny tells him.
Adam comes into the room searching for them. It turns out the list of staff names Hawkes gave him to check didn't turn out anything; but he established the last person in Nicole's room was an ICU nurse. They still have no suspects.
A fingerprint in a bar of soap found by Stella may present another lead. She tells Mac they're searching the database, and he tells her something that's apparently intriguing him: when Nicole came out of the coma, she tried to squeeze her doctor's fingers, but not her mother's. The soap bar print leads to young Matt Huxley, who's also a likely candidate for the doctor's lounge break-in. Mac and Flack find Matt working in a repair shop; the kid tries to run, forcing them to pursuit, running and jumping fences. When they catch him, Flack finds a set of break-in tools in his pocket. Matt states he didn't kill Nicole, at which point the detectives exchange a knowing look. "What makes you think we're here for that?", asks Mac. Matt says he knows they came after him because he was "sloppy" and left his prints behind. According to him, the reason he had to break in was because Nicole's mother didn't like him, and he broke in just to see her. He admits he went to the hospital because, after a friend's accident, he and Nicole had promised each other to "take care of business" if one of them became paralyzed; but he assures the detectives when the time came, he couldn't do it. If Nicole made that deal with Matt, she could have done it with somebody else as well, reasons Mac thoughtfully.
Going back to the evidence, Mac examines Nicole's ECG, noticing a fluctuation by the time Matt says he was there; it indicates she knew there was somebody in the room. He and Hawkes discuss the pact Nicole and Matt made, confronting the idea of a dignified death and the possibility Nicole would not want to live in a wheelchair. Mac points out to the ECG again, showing that Nicole's her heart rate went down right before she died, as if something was calming her; she might have known her killer. At time of death, the ECG shows signs of anxiety, but nothing to indicate Nicole was fighting for her life.
At the lab, Stella discovers a plastic bag was used to smother Nicole. Hawkes finds in imprint of a St. Christopher's medal on the right palm bandage, with traces of flour. Stella recalls seeing Nicole's father with a medal like that in the hospital; but Frank Russo was not on the list of people who had access to Nicole's room. Stella and Flack find Frank Russo at work – in a bakery – and he confesses he sneaked into her room with the help of one of the nurses to say good-bye, since the thought she was not going to wake up. He took her a teddy bear from the hospital gift shop, held the medal in her hand and prayed; that was it. Stella gives him the heart breaking news: the bag the bear came in was the murder weapon. She suggests he might have been the one responsible for taking her life; he strongly denies it. Frank Russo tells the detectives he hadn't seen his daughter in a long time, for her mother had custody and the family moved a lot. "Imagine that", he tells them, "finding out your daughter's in a coma next to an ad for big screen TVs."
Back at the lab, Hawkes and Lindsay examine Nicole's ECG pads; there's four of them, when there should be only three. Mac pulled the three from Nicole's body – where did the fourth came from? At the same time, Flack shows Stella they might have found a break through in the case: Nicole's mother had an insurance policy on her daughter. That's motive. DNA analysis shows another person was wearing the fourth pad – a woman. That's the solution of the ECG mystery – Nicole's heart rate never indicated she was fighting for her life because it actually is the killer's heart rate; she swapped the leads on the pads.
A mother with a motive and a female killer equals Ellen Garner in the interview room. Nicole's friends told the detectives she had had a fight with her mother, Flack points out. Stella reminds Ellen she spent most of the year traveling and left her daughter behind. Ellen Garner tells about Nicole's rebellion, her drug use – "Nicole was self-destructive", she says. However, she claims they loved each other; Nicole's lack of reaction to her at the hospital makes no sense to her either. Flack tries the assisted suicide angle, suggesting Ellen may have killed Nicole because her daughter was so active, how was someone who couldn't "stay still for two minutes" spend the rest of her life in a wheelchair? Ellen assures them she'd never do it; Stella asks her for a DNA sample to prove it.
Behind the two-way mirror, Mac and Hawkes observe. Mac tells Hawkes his father died of small-cell lung cancer; when the pain medication was not working anymore, he asked his son to "end it for him"; Mac refused. Hawkes tells him he made the right decision.
DNA results come as a revolution: Ellen is not Nicole's mother, and as it turns out, Frank is not her father; but there's no register of Nicole being adopted. Besides, the DNA found in the 4th pad shows the killer and Nicole are maternal relatives. Thus comes the astounding conclusion: "What if Nicole isn't Nicole?", asks Mac.
They look at a picture of Nicole Garner and Heather Collins, the other girl in the car accident. They look enough alike that there could have been a mix up at the scene. "That explains why Nicole didn't react to Ellen in the hospital", states Mac, answering yet another mystery. To prove the girls' identities were mixed up, Stella and Mac go back to the crashed car. Nicole was listed as the driver, and Heather as passenger. Blood samples are taken from the car to verify that.
Mac and Stella take Julie Rollins to the interview room and tell her they found her fingerprints on the monitoring equipment. Julie does not try to deny it: "Nicole took my daughter's life", she says. She tells them she was hoping Nicole would never wake up, that she would die, like Heather; when Nicole woke up, she couldn't let her live. Julie Rollins is ready to be arrested, but Mac and Stella have something more to tell her: the person she smothered was not Nicole, it was Heather – her own daughter. Evidence shows that at the night of the accident, Nicole was too drunk to drive, but Heather had drank much less; since she had no driver's license, she put Nicole's on her back pocket, where the paramedics found it and used it to identify her. The accident was Heather's fault; she was the one driving. Breaking down, Julie realizes her daughter tried to tell her, but she thought it was Nicole calling her mother and didn't listen.
As an epilogue, we see Hawkes walking down the street. He passes by an accident, sees a guy who's been hit by a car and is not breathing. Hawkes helps the guy, and when the paramedics arrive, one of them tells him he saved the guy's life. "Are you a doctor?", he asks. "Yeah", answers Hawkes, after a meaningful pause.