At a Renaissance fair, the "king and queen," Miles and Shannon, hold court. They call on Sir William, a "knight," to fight the Black Knight who is twice his size. He takes on the giant but then falls. William's eyes fill with blood.
Meanwhile, House is nude in the apartment nude when Wilson's ex wife Sam walks in on him. He is surprised to see her but doesn't hide himself.
The team reviews William's case and House enters with the man's sword. House has the doctors check out the grounds. House barges into Wilson's office with the sword asking about Sam. He is sure Wilson is making a mistake with his ex.
With an MRI, Taub and Chase find no signs of hematoma in William's brain. He tells them that he is not pretending to be a knight, he and his compatriots live by the Renaissance code of honor all the time. William pukes in the MRI machine.
Foreman and Thirteen pay a visit to William's medieval village community and check out his tent. Miles and Shannon let them know that William challenged other knights to eat strange foods, such as a cow's eyeball. back at the hospital, the team now considers bacterial food poisoning. They perform allergy tests on William. He has a severe allergic reaction, heart problems and a rash on chest. They think it is a lethal bacterial Methicillin resistant staphylococcus aureus (MRSA) infection, yet Foreman didn't use gloves when testing the patient.
House visits Cuddy about Wilson. He wants her to break them up. She refuses to get involved.
Shannon visits William in her normal clothes. His chest pain gets worse. Later he admits to Thirteen that he is in love with Shannon, but that she is engaged to his friend Miles. A true knight would never steal another's bride.
With William's infection spreading, Chase and Taub find no strain of MRSA but it is some sort of organism. House says it is poison ivy, and that the seizures and chest pains are from the community's bonfires. They are paged when William's heart rate slows. Although they suspect he is allergic to epinephrine, Chase decides to inject him to save the heart. William flatlines but returns to normal. The team runs through environmental toxin diagnoses, such as Trichinosis.
Thirteen takes a sample of William's muscle tissue. Thirteen advises him to go after Shannon because it seems like she wants him to.
Wilson tells House that Sam wants to get to know him better. Wilson invites him to dinner with them, and House brings a transvestite as his date. Yet, much to House's dismay, the date gets along well with Sam and Wilson.
Thirteen and Foreman question what they have missed at the medieval village. Foreman wants to retest everything that they previously ruled out in order to appease House who refuses to accept that it's anything other than environmental causes. William's legs suddenly feel like they are on fire and he is put on dialysis. The team tries to convince House that it's rhabdomyolysis and they should be treating his kidneys for cancer. Yet House demands that they investigate William's apartment even though he hasn't been there in weeks.
Wilson approaches Cuddy in the cafeteria because he wants to talk about how well it's going with Sam even though their past was turbulent. Cuddy is hesitant to respond about House's behavior and Wilson's rekindled love affair.
On the ultrasound, Taub finds dozens of tumors in William's liver that are exploding. Yet one cyst pops like a balloon, which is not cancer.
Thirteen and Chase investigate William's apartment and find a secret witchcraft room. House suggests lead poisoning based on the patient's dabbling in alchemy.
House surprises Wilson and Sam by cooking them a gourmet meal and the three seem to be having a glorious time. Yet when Wilson leaves the room, House tells Sam she is the enemy for the pain she's caused Wilson in the past. House will not let her do it again. Sam admits that she hates House as well. They play nice when Wilson returns to the table.
No lead is found in William's blood or liver, but his health is worsening. Thirteen and House go undercover in costume to the Renaissance village looking for something they missed. Thirteen divulges to House that she agrees that Wilson is being stupid about his ex, but he'll need to figure it out himself. House smells something and looks around the apothecary shop. He finds hemlock and learns it was sold to the king, Miles.
Back at the hospital, House confronts Miles about poisoning William in front of Shannon. He swears he didn't. William believes Miles because he's "the best Knight I've ever known." Yet even with treatment for the hemlock, William's heart rate and blood pressure drops. It is not the hemlock that is killing him.
Lucas visits House with his investigative report on Sam. He's found nothing wrong but gives House an envelope of Sam's therapist notes, which he cannot bring himself to read. Sam comes to the office to make peace with House. She asks if he will give her a chance to find out where this relationship with Wilson is heading.
Holding William's hefty sword, House has a realization. The lesions on William's liver indicate an infection on the heart valve, which is a symptom of anabolic steroid abuse. The ancient poison accelerated the damage of the steroids.
Thirteen again advises William to tell Shannon how he feels. He thinks she deserves better, even if it means he is not happy.
House looks at the envelope Lucas delivered with Sam's therapist notes. House tosses it in the trash without opening it.