As a man in his thirties walks along the street, he is mugged by two young men. Although the man tries to defend himself, he starts to become very sick. One of the hoodlums uses the man's phone to call 911.
At the hospital, House is lecturing the Fellows that the victim's respiratory failure is like a "cat burglar." Cuddy walks in with Foreman to show House that he has returned. Cuddy tells the remaining Fellows that Foreman will be her eyes and ears. House announces that he will now make Foreman miserable. Foreman responds that he already is.
The mugging victim who suffers from memory loss has no ID on him, so the doctors name him "Mr. X." Amber and Brennan stress the patient on a treadmill, which causes him to have abdominal pain and tingling limbs. The team tries to unify the symptoms including laryngospasm.
Chase takes bets on which Fellows will get fired. Foreman wants to quit but no one will hire him now. House plans to humiliate Cuddy until she fires Foreman. Suddenly, the patient crashes. Lying unconscious on the floor, Mr. X is wearing a doctor's coat. Foreman thinks he must be faking. He might be suffering from Munchausen's Syndrome and copying the symptoms of neighboring patients. House deduces that it is actually Giovannini Mirror Syndrome, where one innocently picks up the attributes and senses inner thoughts of others to cope with memory loss.
To prove that X is not faking, House has the man join him in a surgery. In the operating room, X is about to perform a surgical cut on the patient when House stops him. House pulls off X's glove, slices his hand and finds that the blood is frozen. House knows the man cannot fake that.
On the way to the cafeteria, House tells the Fellows that the effect of the temperature change on X's blood requires testing for an infection. He will need an ultrasound, blood cultures and a medical history. In order to punish Cuddy for hiring Foreman, House announces that there is tainted mayonnaise in the cafeteria. He suggests every customer go complain to Cuddy.
They requires some clues from X's life. With the patient's car key, House sends Cole and Thirteen to search X's car. Amber draws blood from Mr. X and he mimics her. She gets a look at herself through his eyes. Taub performs an ultrasound and X emulates him as well. Taub finds a lesion on his liver.
Wilson informs House that Giovannini patients mimic whomever they think is in charge. Cuddy sends the Fellows to the clinic to treat the hospital cafeteria's food poisoning victims. House comes to the clinic to retrieve his doctors and asks them about the liver lesions. He then calls out tests for all the uninsured patients in order to upset Cuddy.
Brennan gives Mr. X an ultrasound and finds fungal black pus. This is just like what he found in survivors of the recent tsunami. Brennan tells House that Mr. X has helped him realize that he wants to go back overseas. House rejects this. Amber and Kutner discover that Mr. X is covered in a rash. They put him in a whirlpool to circulate his blood.
Brennan is wrong about his career change and the fungus. Antibiotics are not working, so House needs more information on the patient. Thirteen and Cole search for the car but it has been towed. They drug the guard dogs at the tow yard.
House wants more blood cultures and more accurate patient history. The Fellows respond to House over Foreman. Foreman turns to Cameron, who insists that he belongs with House. Kutner takes spinal fluid from X to find the places the man has lived. In mimicking Kutner, X says that he likes hot tubs. Wilson catches Cuddy switching House's pills with laxatives. Wilson advises her to play up to House's ego instead.
Mr. X's spinal fluid shows he has been to Ohio, California and Central America. Although he complains of having laxative "issues," House suggests heating X from the inside with medication to cause overheating by inducing a high fever. Foreman follows him into the men's room to challenge this.
Cuddy feigns protest to the treatment in order to get House to think he has won. House sees through her charade and says that he knows his pills have been switched. He then lets it slip that perhaps her birth control pills have also been swapped. While Foreman attends to him in the hot tub, Mr. X mirrors Foreman. He collapses from the heat. The doctors dry the patient off and Kutner uses the paddles on him to induce cardiac arrest. Kutner shocks himself because X is not completely dry.
Mr. X continues to suffer, and the team tries to figure out the cause. Foreman thinks there is an infection in his heart which requires a biopsy. House informs Foreman that he has set up another job for him, but Foreman declines. Thirteen and Cole return with items from the patient's car. Knowing that Thirteen didn't want Mr. X to mirror her, House tries to get the man to imitate her. Yet X instead responds as House. Kutner failed to report that X had a memory about himself in the hot tub. The rash returns.
House dresses in Mr. X's clothes and reflects back to himself using things from the car in order to get X to reveal more about his life history. X uses the vapor rub found in the car to mask the smell of "dung." House realizes the problem. Mr. X is a farm equipment salesman from Ohio and has contracted an infection from pig feces called eperythrozoon.
Before they start treatment on the patient, Foreman wants X to do one more thing. House and Cuddy present themselves to X to see who is more dominant. Wilson, Foreman and the team watch. Wilson tells Foreman that House was lying about the job. Mr. X observes and mimics House and Cuddy. House is the more dominant.
Flush with victory, House lets the team know that none of the Fellows are fired because they all made mistakes. Chase wins his bet. Foreman is aware that Chase was splitting his winnings with House, which is why no one is fired. Foreman is staying.