Two teenagers are preparing to mug a man as he comes out of a bar. They approach him but he snaps at them. When one of them puts a knife to his throat, he starts coughing. One runs off and the other one stops to call 911.
House is running a differential on the mystery man with his fellowship applicants when Cuddy brings in Foreman and announces he'll be supervising the group. House is less then thrilled and orders exercise test to invoke Mr. X's breathing problems again, then threatens to humiliate Foreman until he quits again. The applicants discover that Chase is running a pool on which of them will drop out next. Mr. X has stomach pains and foot tingling on the treadmill, then collapses. House disrupts Foreman's attempts to run differential with the fellowship applicants but then they get word Mr. X has collapsed. Foreman realizes that Mr. X is emulating the symptoms of the patients around him, and House believes he has Mirror Syndrome, mirroring the characteristics and symptoms of anyone he sees.
To test his theory, House takes the patient into the operating room where Wilson is performing surgery. As Wilson starts to berate House for giving Foreman a hard time, Mr. X starts doing the same thing, then tries to operate. House pulls off the man's glove and realizes the man's blood is coagulating in his veins.
House still needs to find the cause of the man's primary problem and checks for infection. He then tries to get everyone out of the cafeteria with a mayonnaise panic over Foreman's objections to annoy Cuddy, and sends them to check Mr. X's car keys. House sends Cole to check them out and Thirteen volunteers to go with him. Amber and Taub administer tests and Mr. X mirrors their personalities, echoing Amber's insecurities and Taub's attractiveness toward her. Taub finds a lesion on the liver and House goes to Wilson, who reveals he has been reading up on Mirror Syndrome. They tend to echo authority figures, and Wilson is amused that Mr. X mirrored him. House finds that Cuddy has all of his applicants in the clinic and House tries to run a differential. He then orders complete testing for all the uninsured patients.
Brennan runs a test on Mr. X and discovers he has a black pus that may be fungus-related, while the patient notes he's unhappy there. Brennan goes to House to resign but House convinces him not to. Mr. X's blood continues to coagulate and they immerse him in hot water and go back to differential. House calls to get an accurate history and talks to Cole and Thirteen, who are breaking into the impound lot where Mr. X's car is held. While they drug the guard dogs, House sends the others out… all ignoring Foreman except Taub, who notes that Cuddy will give in first.
Foreman talks to Cameron who notes that he's always been happy working with House no matter how much he objects. Meanwhile, Mr. X mirrors Kutner's desires for thrills, and Wilson goes to Cuddy who is planning to get revenge on House with the use of a laxative substituted for his Vicodin. Wilson suggests she make House think he's won, but it's too late as House emerges from the restroom. Kutner warns that Mr. X's rash is back and the hot water is working, but House has to head back to the restroom and spars with Foreman. House then decides to induce a fever to keep the symptoms in check, and Cuddy agrees. House is suspicious of how easily she gave in and warns that he isn't giving in… and she might want to keep an eye on her birth control pills.
Foreman gives Mr. X a fever but he has a heart attack and they get him out of the tub. They dry him off barely in time as Kutner administers defib and shocks himself unconscious. House is interested in why Kutner is interested in suffering pain, and Foreman suggests they do a biopsy on his heart. When they point out it won't work, Foreman suggests they biopsy his toe. House draws him off to the side to tell him he's found Foreman a job since nobody's going to back down. Foreman turns down the offer and seems happy at the thought of staying.
Cole and Thirteen have found Mr. X's car and have a name, Robert Elliott, but they don't have enough time to track down his history. House wants to know why Thirteen volunteered and suggests she didn't want to have Mr. X mirror her. He gets Foreman out of the room and has Thirteen meet with the patient. Robert says she's incredibly hot, mirroring House. House steps outside while Thirteen runs the test and Robert admits he's scared. She comes out and House wants to know how he reacted. House agrees with Kutner's suggestion to immerse him in hot water again, but then picks up on the fact that Kutner doesn't like hot tubs. House realizes that Robert reacted on his own to the hot water.
House then goes into the room dressed in Robert's clothing and claims to be him, and Robert responds to him. House then goes over the restaurants he's visited and the fact he has a tube of vaporub. Robert puts it under his nostrils and says it covers the smell of dung. House tells the others that Robert sells farm equipment and got an infection from pig manure. Foreman suggests they wait 15 minutes and have Cuddy and House meet with Robert at the same time to see which one is the dominant personality. Robert tells both of them to shut up… then starts mirroring House.
House meets with all of the applicants and declares it a tie, firing none of them. Foreman notices it was nice of House and Chase won all the money. It turns out House split the money with Chase and bribes Foreman with letting him stay so he doesn't reveal what was going on. House then notes that everyone reacted to Robert but no one changed… except Foreman.