Six-year-old Emily and her father Simon are at the park playing and she insists on going on the merry-go-round by herself. Simon makes her promise not to tell her mother and then takes video as the merry-go-round starts up. However, Emily starts bleeding from the nose and then falls off of the horse, and Simon yells at them to stop the ride. He jumps onboard and finally finds her lying in the center, unconscious.
Wilson is looking at the scans of his tumor when House turns up at the office where he's getting treatment. House insists on staying, pointing out all the times that Wilson wouldn't leave him alone and promises not tell anyone. Wilson gives in and House pours himself a drink to celebrate, explaining that he's on vacation
At the hospital, Taub figures that it's a sign of the Apocalypse that House is taking a vacation. Foreman tells the team that they have a case of Emily, who was born with a rare mutation of A-T. The doctor with him is Dr. Elizabeth Lawson, a developmental geneticist who is an expert on Emily's condition. Chase points out that Emily's last name is also Lawson and Elizabeth admits that she's the patient's mother, but she's there as a doctor. She insist it's not a natural progression and explains that her lungs are functioning normally. Elizabeth disagrees with their diagnoses immediately, saying she has already considered the easy solutions and calling Foreman by his first name. Park suggests Wegener's granulomatosis and Elizabeth agrees that it's a good fit, but warns that they can't do x-rays because the A-T makes Emily sensitive to radiation. As Elizabeth goes to prep the MRI, Foreman tells the team that she'll be an asset and makes it clear that it's an order, not a request.
Wilson's oncologist, Dr. Michael A. Kondo, warns him that the tumor has spread to the surrounding tissue and that they'll have to shrink it before they remove it. He says that they'll use radiation treatment three days a week and then go to chemo, but Wilson points out that if it hasn't shrunk by that point he'll be dead. Kondo tries to reassure him, saying they caught in in time, but Wilson wants the radiation and chemo concurrently. When Kondo refuses, Wilson says that he's going to get a second opinion and walks out. House watches the entire thing and Kondo tells him to talk to Wilson, but House tells him that Wilson is right.
At Emily's room, Elizabeth complains about Simon taking Emily on the merry-go-round, saying that she has balance issues. Taub and Adams arrive and Elizabeth explains that Simon had her for the week when Emily collapsed. Elizabeth wakes her daughter up and tells them that they're taking her for a MRI.
House and Wilson play Beer Battleship and House finally asks why his friend freaked out with Kondo. Wilson says that he wasn't freaking out but House says that Kondo was right. He warns him that every day that Wilson waits is another day that the tumor gets larger. Wilson says that he's going off on his own and walks away.
As Adams and Taub run the MRI, Emily tells them that she was just pretending to be asleep earlier to avoid her parents fighting over her. Taub says that his children do the same thing, but avoids saying that he's divorced. Adams comments that Elizabeth refers to her daughter as "the patient," and Taub sympathizes. Emily starts crying in pain and they run in and discover that she's suffering from necrosis of the fingertips.
Back in differential, Taub says that they restored circulation and Adams suggest stress. She tells Elizabeth that Emily heard her parents fighting, but Elizabeth says that her daughter has heard worse, ruling out stress. She then suggests heavy metal poisoning, noting that Simon moved into an old apartment building three weeks ago. Elizabeth says that Simon is a good father but careless with his daughter's health. Chase says that lupus is a better fit, but Elizabeth pulls rank and exerts her authority as the parent, ordering chelation therapy. Once she leaves, Chase says that they should treat her like any other mother.
Chase and Adams break into Elizabeth's house and discover that she has medical equipment and drugs in her basement. They discover that Elizabeth is using LEX-2, an experimental antibiotic that can override DNA errors, and Chase suspects that she's administering it to her daughter. Adams isn't convinced but Chase points out that there aren't any lab rats.
House comes to see Wilson in his office and tells him that he's been checking. Wilson has been to seen four other doctors and in each case wanted chemo doses so high that there's a one in three chance it could kill him outright. House figure Wilson didn't tell him because he already found someone insane or stupid enough to give him the potentially lethal dose of chemicals. He's also figured that Wilson is ready to administer it to himself and finds the box of drugs in the office. Wilson says that he was going to do it at his place and House prepares to cut open the chemo bags. His friend tells him to stop, saying that he's still healthy and he wants to go the extreme route now when he has a better chance of surviving it. House points out that the treatment is just as lethal as the tumor, and Wilson tells him about all of his patients that had a high chance of survival and still died. He insists that he won't die slowly in a hospital bed no matter how great the chance he'll survive. House tells him that he's an idiot... and they'll do it at House's apartment.
Chase takes the LEX-2 to Foreman, who confronts Elizabeth. She insists that she tested the drug on herself first and didn't document any adverse effects. Foreman points out that a trial of one is irrelevant but Elizabeth explains that LEX-2 won't be approved for at least five years. Without the drug, Emily would already be dead. Foreman has checked with another researcher and confirmed that the drug has been linked to renal failure in lab rats. They'll need to do an ultrasound renal biopsy to check for damage, and Chase hopes that it's reversible as Elizabeth stares in shock.
Wilson and House prepare to administer the chemo and House proposes a toast. He describes every excruciating symptom that his friend will suffer and then asks what they're doing there. After a moment, Wilson hands him the chemo bag.
Chase does the ultrasound biopsy as Simon looks on and assures Emily that she's doing fine. Elizabeth watches as Emily complains of chest pains and starts coughing up blood. Chase confirms that it isn't her kidneys.
House gets Wilson some soup and Wilson thanks him for taking the risk. His friend assures him that he's already picked out a spot to dump the body if things go wrong, and notes that Wilson is already suffering muscle spasms ahead of schedule. As they continue the treatment, Wilson admits that he thought that he'd have a family to look after him when he got old and sick. House points out that they both have everything they need: industrial strength painkiller. He administers it to Wilson and himself at the same time and tells his friend that they're on vacation.
At the hospital, Chase tells the team that he figures they're looking at side effects from the LEX-2. Taub suggests a clot from Emily's fall and Elizabeth immediately orders heparin in case her daughter suffers an embolism. The team gets a page that Simon is checking Emily out. They go to the lobby and discover that Simon is taking Emily to a different hospital. He points out that everything they've done has failed and that Elizabeth was the one who treated their daughter like a lab rat. Chase tells them to stop fighting as the girl clamps her hands over her ears. They tell Emily that they're just going to talk and Chase takes her to get some candy. Taub tells Simon about their new diagnosis and warns that Emily could die before the new doctors are up to speed. When Taub tells Simon that the clot was caused by the fall on the carousel, he blames himself. Elizabeth says they should stop blaming each other but Simon angrily points out that she knows nothing about Emily other than what she's seen when she had the girl in her basement. In return, Elizabeth asks Simon what he knows about her medical condition and says that if she can't find a cure then their daughter will die. Simon asks her to make Emily better and walks away.
Wilson wakes up and finds a boy sitting next to him. There's no sign of House and the boy says that he just walked in. He's playing with a toy that belonged to one of his patients and Wilson realizes it's John Taylor, a child that he promised would live. John asks why he died if he didn't do anything wrong, and Wilson wonders if he's dead. He wakes up and House tells him that it just feels that way.
Taub and Park run another MRI as Elizabeth looks on. He explains that he knows that it's easy to forget what matters. Elizabeth says that she hasn't. They confirm that there's no clot, but Emily's eyes are jaundiced, meaning her liver's failing. Back in differential, the team confirms that they found a blockage restricting blood flow to her liver. Elizabeth suggests that they're seeing the end result of her condition and says that she shouldn't be there. She walks out of the conference room and the team tries to work out what's wrong. Taub suggests advanced Lyme disease and Chase notes that they wouldn't pick it up if it were advanced. However, that doesn't explain why it suddenly became active. Adams suggests that Emily has changed since Elizabeth stopped giving her the LEX-2. It was keeping the Lyme in check, and they start her on amoxicillin.
Wilson continues suffering as House tends to him. He informs Wilson that his white cell count is dropping and says that they're out of morphine. Wilson realizes that House has been using his personal stash of painkillers and wonders if he has enough for both of them. House gives him the pills without comment and then limps out to condition and counts his few remaining pills. He then drinks some whiskey instead.
Elizabeth holds Emily's hand while Simon sits nearby. Chase and Adams come in and explain their diagnosis, but Elizabeth says that there's no chance she could have gotten Lyme. Emily admits that she went into the woods with Simon, who explains that he took her to take photos in the forest. Elizabeth admits that they can't protect her from everything and calmly says that they need a lumbar puncture to confirm.
Wilson finally pulls himself off of the medical sensors and crawls across the floor to the bathroom. He refuses to let House change his adult diapers and admits that he's pathetic. Wilson realize he was telling his patient not to bother finding an answer, and figures he should have been a self-centered manipulative ass like House... because then he would have felt like he deserved cancer.
As Chase and Adams do the LP, Emily wonders if her parents will get back together when she dies. She complains about paralysis of the arm and Chase confirms that she's having a stroke.
Wilson's white cell count drops and House says that they have to go to the hospital now. Wilson holds his friend to his promise, insisting that he'd rather die in the apartment than in an ambulance or a hospital. Crying, he begs House not to do it to him and makes him promise. After a moment, House promises.
The team goes over the new symptom and try to put things together. They wonder what could mimic an embolism and worry that House isn't there to help. Park suggests cancer and Taub points out that cancer wouldn't present in three days. Chase says that it's an atrial myxoma, a tumor that is blocking blood flow and causing the clots and all of the symptoms. Chase tells Elizabeth and Simon that they can remove the tumor surgically and hope that it's benign.
The next morning, Wilson wakes up and House offers him a glass of water. He tells his friend that they can wait seven to ten days for the swelling to go down, rescan, and schedule the surgery to remove the shrunken tumor. Wilson tries to apologize for what he said but House claims that he ignored it all and complains about his leg pain. He then helps Wilson to the bathroom, and Wilson realizes that House gave up his Vicodin for him. He also realizes that what he felt like is what House goes through all the time, but House admits that at least he doesn't have cancer.
Chase tells the Lawsons that the tumor was benign and they removed all of it. Emily wonders if she's okay and Elizabeth admits that she's still sick because of her genetic condition. Simon says that it's never stopped them before and plans a trip to the aquarium, and Elizabeth asks if she can go too.
Wilson and House arrive at the hospital and Wilson says that he'll see him at lunch. He goes to his office and discovers that House has sent him photos of Wilson unconscious from the chemo, posed with prostitutes and booze.