A cop named Joe Luria corners a young gang member in an alley. Joe giddily mocks the perp as he reads him his rights. The gang member pulls his gun and shoots. The bullet shatters against Joe's flak jacket, with a piece deflecting up through Joe's neck and into brain matter at the base of his skull. Joe lies on the ground, laughing as blood gushes from the wound.
House and the team deduce the cause of Joe's hysterical reaction. Chase thinks that the bullet fragments in the brain are to blame, but House points out that it is the wrong area to cause euphoria. They will need to expand their search, factoring in Joe's cough and cloudy lungs.
Chase mentions carbon monoxide poisoning, which would explain the elevated heart rate, coughing and impaired neurological functions. House considers that the patient might have been exposed to CO indoors and went outdoors before collapsing. He orders an arterial blood gas test. In the meantime, they must check Joe's squad car, personal car, precinct and home for gas leaks.
Chase finds low-level CO poisoning. He is about to slide Joe into the hyperbaric chamber when Joe's fist suddenly clenches. As his brain struggles for oxygen, Joe loses motor function. That grim news can't take away Joe's giddiness. Yet when Cameron mentions that someone is checking Joe's home for a gas leak, he immediately turns serious.
Foreman searches Joe's incredibly filthy apartment for some clues. He swabs samples from the rank kitchen. Foreman then steps through the window onto the building's roof and notices a shed with a power supply. He finds a hydroponic marijuana farm.
House goes to the precinct and hears a cop with a raspy cough. The man's desk is right next to Joe's, below the same air conditioning unit.
Back at the hospital, Foreman is convinced that marijuana is the explanation. House believes that Legionnaire's disease is the cause, citing the rancid water in the AC unit as evidence. The next morning, Joe is feeling better, and Foreman observes that his COHb levels are down. Chase points out on the x-rays that the clouded area in the upper lung lobes are clearing up. Joe seems more concerned with making sure Foreman won't reveal what was at his apartment. Suspecting something is wrong, Foreman spins around the portable light board and shows it to Joe, who agrees that the x-rays look fine. The doctors realize that Joe is blind.
Foreman reports that Joe's papillary responses are intact, the fundus looks normal and there's no macular degeneration. He thinks Joe has Anton's Blindness, a condition in which patients can physically see but the brain cannot process the information. This indicates damage to both occipital lobes. A stroke is a possible explanation. House suspects a brain clot, but they can't do an MRI because the bullet fragments will move and shred Joe's brain. Cameron suggests an angio x-ray. Although House considers this a waste of time, the team badgers him into it.
Cameron explains to Joe that they will send a catheter through his femoral artery to his brain. Foreman remarks to him that he'll be back on the streets scaring people. When the team reconvenes in the morgue with the results, Cameron presses House to remove Foreman from the case because he hates cops. Foreman says he was just having fun with a hypocrite, so House lets him stay. There is also the fact that Foreman is the team neurologist.
The angio shows some clotting, but not enough to be decisive. House again suggests an MRI, which Foreman again shoots down. House pulls out a gun and shoots a cadaver with an identical bullet. They can now run a test MRI to see how the bullet is affected. Cameron and Chase are shocked and scared, while Foreman is merely bemused.
Cameron continues to harp on Foreman's behavior. House asks whether it was aggressive or giddy, noting that Foreman's amusement at the gunshot isn't a normal reaction. Foreman adamantly insists that being bored by House's insanity isn't proof of illness. With the cadaver in place, House flicks the switch on the MRI. The bullets are immediately ripped out of the skull and forever buried in the magnetic coils.
They learn that the MRI is out of commission for at least two weeks. Foreman wonders if doing nothing is their only option, seeing as how the giddiness seems to have disappeared. The blindness hasn't, so House orders an echo of Joe's heart to search for the source of the clots. They could get lucky.
As Cameron and Chase perform the ultrasound, Joe goes into tachycardia. They rush to save him, while Foreman merely stands back and giggles. Chase recognizes intracranial bleeding, forcing them to cut Joe's temple to relieve the pressure. Foreman can't stop laughing.
Foreman is sealed in an airtight bio-safety room with Joe and two nurses wearing full biohazard suits. He still insists that he's fine, but House is more focused on finally getting a chance to use an MRI to locate the problem. They will use it on Foreman.
House draws his own blood sample and informs Chase and Cameron that anybody with an elevated SED rate is joining Foreman. He has noticed in the MRI an area of increased T2 attenuation in the cingulated cortex. This mushiness would explain the euphoria, but what explains the mushiness? House asks who wants to investigate Joe's apartment next. Cameron turns to leave, but House stops her. Foreman brought back samples from the apartment. House was merely testing them.
Cameron sorts through the samples using protective gloves built into a protective steel case. At the same time, Chase tries to draw blood from Foreman. Foreman asks Chase what they're thinking because he believes it might be a staph infection. If Chase delivers linezolid directly into their brains, Foreman and Joe can be cured.
The samples test negative for toluene, arsenic and lead, and the blood is negative for West Nile or Eastern equine diseases. Cameron wants to go to the apartment for more samples, but House refuses to allow it. He wants to take a sample from Joe's brain, but surgery is impossible because he is on blood thinners. Using Foreman is the only option. Chase tries to resist with everything Foreman told him earlier. Yet House knows where Chase is getting this line of thought.
House heads down to the isolation chamber to talk to Foreman directly. House doubts that he has a staph infection because it would present in numerous different ways before a brain abscess. House offers Foreman a release to sign so he can biopsy his brain, but Foreman wants to see the MRI first. He insists that the mushy spot on the x-ray could have developed into an abscess by now. House mentions fever and Foreman's reads 101.6. Foreman insists that House put an omaya reservoir into his skull and treat him for staph.
A neurosurgeon drills into Foreman's skull, exposing his brain. Foreman, wide awake during the procedure, looks at flash cards for Chase and identifies the simple shapes on each one. Foreman then hears House's voice coming from behind his head and realizes what's going on. House is going to biopsy his brain. Foreman orders him to get out of his temporal lobe.
Foreman wakes in the middle of the night, back in the isolation room. Joe says he can't see anything, and Foreman is encouraged by this because Joe is now aware of his blindness.
The biopsy shows non-specific signs of inflammation. Cameron quickly points out that House's “can't miss” idea stole a billion of Foreman's brain cells, turning up nothing. Yet the biopsy was also negative for a staph infection. Cameron again asks to go into the apartment. House turns her down once more. They will instead retest the samples for any toxin, bacteria or fungus that attacks the brain. House orders Cameron to suit up to monitor Foreman for Anton's Blindness. They need to track Foreman to see how far behind he is from Joe.
Wilson questions why House is being so cautious and avoiding Joe's apartment. House doesn't want to lose another doctor. Wilson realizes that Foreman is not simply another patient to House, no matter what he claims.
As Joe writhes in agony, Cameron tells Foreman they found nothing in his brain. Foreman suggests returning to the apartment because he might have missed something. The cause may be listeriosis. Cameron says that they cannot go back because of the danger. Foreman becomes angry. He picks up a syringe he used to draw his own blood and jabs Cameron in the leg. He says she can either tell House what happened or head to Joe's apartment to save all three of them.
House and Chase stand outside the chamber as Foreman throws out possible diseases to them. Joe continues screaming in pain, so Foreman picks up a syringe and injects morphine into his IV. Chase yells that Joe is already at his daily limit and more could kill him. Realizing that the pain could cause a stress cardiomyopathy, House makes no attempt to stop Foreman. The screaming continues, and the doctors realize that Joe has a new symptom -- hyperalgesia. The infection has spread to the pain center of the brain, which is telling Joe that his entire body is in tremendous pain. No amount of medicine can soothe it. House tells Chase to suit up and induce Joe into a coma.
Foreman continues to throw out explanations to House, who wonders why Foreman isn't concerned that Cameron is missing. When Foreman doesn't react, House starts to figure out where she is.
Cameron samples Joe's entire apartment, including his rooftop farm. As she is re-sealing the biohazard tape on the door, she turns and finds House. Cameron tells him about the needle, and House can't believe she came to the apartment instead of killing him on the spot. Even by breaking the skin, the chances of infection were remote. Cameron wanted to be here.
House roots through Cameron's samples. He's disinterested by the normal garbage, but his curiosity is piqued by the inclusion of three loaves of rye bread. He sends Cameron back inside. Using his cell phone, he directs her out onto the roof with the bread in order to draw out pigeons. He instructs Cameron to look for pigeon droppings. She doesn't find any, and House has her look for a dustpan because he figures Joe uses the droppings for fertilizer. She finds a used scraper on a bucket. The bucket full of pigeon droppings is the perfect home for Cryptococcus neoformans. Once that enters the brain, it causes happiness, blindness and intractable pain.
Cameron puts a sample of droppings onto a slide and adds GMS stain. She doesn't get the result she was expecting and sprints upstairs. In the isolation room, Joe crashes. Cameron runs up and tells the team that the sample was negative for Cryptococcus. As the doctors suit up, Foreman shocks Joe with no results. A subsequent epi injection does nothing. Joe dies.