At a medieval reenactment community, modern-day people live life like centuries before. Knights gather on the jousting field as King Miles informs them that the queen will choose their champion from those who will battle for her honor and a title. They must defeat the captain of his guard, Sir Horace the Black. The queen walks among them and asks if Sir William will fight for her honor, and he vows to fight to the death for her. She chooses Sir William, and gives him advice on Horace's weakness while Miles looks on suspiciously. He questions her choice but she insists, and the fight begins. Horace disarms William and knocks him to the ground, but he returns to his feet, recovers his sword, and finally drops Horace. The captain of the guards collapses and the crowd cheers William on. As the queen approaches her chosen knight, William staggers and clutches at his helmet, the whites of his eyes filled with blood. Miles takes out a cell phone and calls an ambulance.
At their condo, House is taking Ibuprofen in the nude when Wilson's ex-wife Sam Carr comes in and says that Wilson was called off to work. She's initially surprised but soon recovers.
At Princeton, the team is considering cases when they see House coming in with a sword. He informs them that they'll be taking William's case and explains that he's part of a troupe that reenact the medieval era. William has been camping out at the fairground for weeks. Foreman believes it's a concussion, but Chase notes that the ER wouldn't have informed House if it were that simple. House describes the bloody eyes and sends Taub and Chase to test for concussions while Foreman and Thirteen go to the fairground to take samples.
First, House stops off to ask Wilson about his new date. He soon realizes that Sam is Wilson's first wife, the "soulless harpy" he never met. Wilson insists he has everything under control and doesn't want House to interfere. House says he'll soon be back to say "I told you so."
Chase and Taub test for a hematoma and discuss William's case. He hears them and insists that he's not pretending. William explains that it's a lifestyle and he and the others live by a code of loyalty and courage, and beat each other up for fun. He complains of nausea and starts vomiting, and they rush to get him out.
Foreman and Thirteen take environmental samples, and they can't resist pointing out Foreman's strange garb. Thirteen tells him to relax and play along, but Foreman would rather focus on the potential for food contamination. King Miles and Queen Shannon come over and ask how William is doing, and they ask to see William's tent. Foreman notices a vomit-covered cow's eye. Miles explains that it's part of an eating ritual he makes the knights perform.
Back at the hospital, House dismisses their theory of food poisoning, noting no one else was sick. He tells them to run scratch tests for allergies and treat him with epinephrine in the meantime. As Foreman runs the test, Shannon visits William, interrupting the preparations for her wedding to Miles. William's heart seizes up and they discover red blisters on his chest, suggesting an allergy to epinephrine. Thirteen suggests staphylococcus that was spread when Foreman treated William without gloves, and could potentially spread throughout the hospital. House then goes to see Cuddy to talk about Wilson. Cuddy already knows about it and House is surprised Wilson was able to keep it a secret so long. He wants to break them up before Sam turns harpy again, warning that Wilson was a wreck afterward. House figures that she'd had the most experience in dating losers, but Cuddy warns that if House forces Wilson to choose, he might not like the answer.
Thirteen checks on William, who asks Shannon to go get him some pop. Once she's gone, William explains that he's in love with Shannon but he doesn't want to break up her relationship with her fiancée, Miles. He prefers to live like a knight as far as honor.
Taub and Chase go over William's test results and confirm it's not MRSA. Chase admits he's impressed by William's bravery. Taub disagrees, and Chase figures he thinks that way because he never won a fight. House arrives to inform them that the blisters are caused by poison ivy, and confirms it by revealing he got a poison ivy rash from handling William's sword. William seizes and Chase manages to revive him using epinephrine, overriding Taub's objections.
The seizure eliminates MRSA as a cause and they conclude there must be some other environmental toxin. House suspects trichinosis and orders treatment and a muscle biopsy to confirm. He then goes to see Wilson, who explains that Sam wants to know House better. Wilson reluctantly agreed because she thinks Wilson was hiding House from her. He was, but he doesn't want Sam to know it. House agrees and Wilson notes that House is worried about him. However, he insists that he doesn't want House's help, and tells him to just come and be his usual selfish stuff.
Shannon comes to see William again, and he says that he doesn't want Miles to see him. He's fine with her seeing him, and Thirteen hastily explains that it's a guy thing so Shannon won't suspect William's feelings. Once Shannon leaves, Thirteen suggests that she's waiting for William to make the first move. She tells him to take what is his, but William says that Shannon isn't his.
Wilson and Sam wait for House to arrive. They're surprised to see House arrive with a transvestite prostitute, Sarah.
Thirteen and Foreman check the muscle biopsy and confirm that it's not trichinosis. They wonder what they missed at the fairground, and Foreman suggests it's a fungal infection. He figures that House is suffering from extra pain, making him extra crane, and they'll have to go to extra lengths to prove it's environmental.
House is upset that Sarah is bonding with Sam and Wilson.
William has sudden shooting pains in his legs. They check and discover that the legs are swelling with blood, indicating kidney failure. Taub suggests cancer and Foreman insists its' not environmental. He irritably tells them to check William's apartment, despite the fact William hasn't been there in weeks. Chase and Thirteen do it while Foreman and Taub check William's liver.
Cuddy talks to Wilson in the cafeteria and he describe what happened. When she doesn't react, Wilson wonders if House said something to him but she denies it. He persists and tries to discuss his relationship, explaining that Sam is less competitive. Wilson asks Cuddy if he's out of his mind, but she remains non-committal, saying he might as well give it a chance. He smiles and agrees.
Foreman and Taub confirm that William's liver is filled with what appear to be tumors. However, when they magnify them on the scanner, they realize that while they can't identify what it is, it's not cancer.
Thirteen and Chase check out William's apartment, which contains medieval model landscapes with miniature figures. They find a locked door and Chase tries to break it down without much success. Thirteen gets it open when she finds the key under the mat. Inside they find a sanctum dedicated to the black arts. They bring his books and potions back to the hospital, and confirm the potions aren't a poison. Taub suggests the black spots are blood lesions, but House notices that the figures from the model are lead and proposes lead poisoning as a diagnosis. The black spots are cysts, releasing the lead into William's bloodstream.
Wilson and Sam return to the condo but she'd rather stay in for sex rather than make the show. However, they hear House singing and discover that he's cooking. He explains that he made dinner for them and his team will handle William. Wilson figures that he's up to something but House insists that he's trying to be nice. He apologizes for bringing Sarah and seems sincere. They have dinner together and things seem to go well. Wilson needs to go the bathroom, but worries that it might be dangerous to leave them alone. Sam and House assure him they'll be fine. Once he leaves, House tells Sam she's a cold-hearted bitch that left Wilson damaged for years, and he doesn't plan to let her do it again. He admits he made dinner to get to know the enemy, and they agree to dislike each other as long as Wilson isn't around. Wilson returns and they go back to pretending to like each other.
The next morning, House returns to his office and the team tells him there's no sign of lead poisoning, and William's heart and BP are growing worse. House decides that they need to go to the fair, in appropriate dress, so they can find whatever is causing William's symptoms. He can't resist discussing Wilson's relationship with Sam, and Thirteen figures that he'll eventually be fine. House warns that Wilson is actually a nice guy, i.e., a sucker, and they have to watch out for him. House smells something from the apothecary shop and goes to investigate. The apothecary is an apprentice and doesn't know much about what's in the shop. House finds hemlock and confirms that the apprentice sold it to... King Miles. House realizes that they checked the environment, but not the people living in it.
Back at the hospital, House confronts Miles and accuses him of poisoning William. Miles denies it at first, and then claims that he bought it for the food challenge. When he wonders what his motive is, Thirteen points out that he has one reason. Shannon tells him to tell the truth, but Miles insists that he didn't do it. Tests confirm that William has hemlock poisoning, and House tells William that Miles poisoned him. William insists that Miles didn't perform him, and Taub notes that the treatment isn't working. Either William would be dead by now, or the treatment would work. Whatever he has, it isn't hemlock intoxication.
Lucas brings House a report on Sam. It shows that she has a typical life with nothing suspicious in her background. Lucas admits that he didn't read her psychiatrist's notes, even though he stole them. Sam comes in and asks to talk to House privately. Once Lucas leaves, Sam says that she understands why House suspects her. House isn't impressed, but Sam says that they need to work together to make Wilson happy. She'd like a chance to find out if she can make a new relationship with Wilson. House doesn't have an answer for her and she leaves. As she leaves, House contemplates William's sword and gets an idea.
House tells the team that the lesions are hepitus lesions, which indicate infection on his heart that in turn indicate steroid abuse. The hemlock accelerated the steroid poisoning. They treat William and Thirteen goes to see him. She wonders why he compromised his ideals for a fight, but not for Shannon. William explains that the fight was a game, but his feelings were real. Thirteen tells him that Shannon takes it seriously, and advises him to tell her how he feels. William explains that Miles is a great guy and Shannon will have a great life with him. However, William can't offer her anything, and he'd rather she be happy. Thirteen notes that he's an idiot.
Later, House considers Sam's psychiatric files, starts to open them, and then throws them away. He then takes more Ibuprofen for the pain.