House admits to Wilson that he made copies of Stacy's psychiatric evaluation. Wilson is aghast at the breach of ethics. House is incredibly intrigued to find out that things are rough between Stacey and Mark. The couple isn't even having sex.
A man named Kalvin Ryan waits outside House's place, and House is quite familiar with the man because he's practically been stalking him. Kalvin begs House to examine his file, but one quick look at the man's appearance is all House needs to declare that it's AIDS. He advises Kalvin to get his T-cell count re-checked, which Kalvin says he's already done.
Kalvin collapses and begins wheezing. Wilson realizes that the patient is going into anaphylactic shock. Later, Stacy informs House that Kalvin has stated that he will press charges for assault is House refuses to treat him. House presents the case to his team. Foreman and Cameron note that, according to his file, Kalvin recently tested negative for TB, PCP, MAC, CMV and HSV. His T-cell count is at 200. They don't agree with House that an infection is a foregone conclusion.
Kalvin's new meds have rebounded his immune system and dropped his viral load. He's getting better, but also getting sicker. House wonders if the new meds woke up Kalvin's immune system to cause an old infection to restart. He orders an x-ray to find the old infection and steroids for treatment.
Trying to impress Stacy, House visits her home to help her kill a rat. While there, he subtly tries to sabotage her marriage by offering observations that just happen to match the thoughts in her psych profile. The doctors call, informing House that Kalvin's lungs are too scarred for any infections to show on the x-ray. House orders a round of meds to treat the last three infections Kalvin was known to have had.
Kalvin coughs blood that splatters across Cameron's face. The hospital's infection control officer informs her that she'll be given post-exposure prophylaxis and three anti-viral medications. She will also be tested for HIV at six weeks, three months and six months. House is more interested in what the cough means because it's a new symptom. He has Chase and Cameron check Kalvin's pills for toxins.
In Kalvin's home, they find no interesting medicines. Yet they do find photographs that Kalvin took of smashed up light bulbs. The bulbs date to the 1930s, when fluorescent lights contained large amounts of beryllium. Beryllium dust inflames the lungs, causing them to be rigid, which makes it difficult to breathe. Cameron creates a small incision on Kalvin's back in order to biopsy his lungs for beryllium exposure.
Meanwhile, House tries to capture Stacy's rat instead of killing it. House noticed that the rat is tilting its head, which indicates an infection or a brain tumor. This could have been caused by something in the house which could affect Mark and Stacy. House begins laying out cheese spiked with antibiotics.
Back at the hospital, Kalvin complains of chest pains and he is struggling to breathe. He is bleeding from his lungs into the area around his heart. Foreman plunges a needle into Kalvin's chest. The syringe begins to fill with a clear fluid instead of blood. This indicates a tumor in Kalvin's heart.
Wilson reports that a CT scan confirmed the tumor, along with several smaller ones in the lungs. He is sure that it is non-Hodgkins lymphoma. Cameron thinks it could be sarcoidosis, and she asks to do the Kveim Siltzbach test. This stops the doctors in their tracks because the test isn't FDA approved. House knows that Cameron can't resist the strained relationship between Kalvin and his father.
That night, Stacy and House stake out the rat. He notes that the rat's urine indicate toxins from cigarette smoke. Stacy admits to blowing her smoke into the vents. The rat then walks right into one of the traps and is caged.
Cameron calls Chase over to her apartment. She had a discussion with Kalvin and he told her that the discovery of his HIV finally allowed him to let go of worrying about what other people thought. Kalvin had needled her about being too uptight. Cameron decides to let loose and experiment. After taking some of Kalvin's meth, she drags Chase into her bedroom for a wild night.
The next morning at the hospital, Cameron and House arrive to find Kalvin arguing with his father. House asks why Kalvin feels like he needs to apologize if the dad is the one who kicked him out. After some heated back and forth, Kalvin's father says that Kalvin killed his mother.
Cameron explains that Kalvin had lied. Mom's kidneys were failing and Kalvin was a match. He was also HIV positive and couldn't donate his organs. House gets the results of the Kveim test which indicate the patient is negative for sarcoidosis. Wilson breaks the news to Kalvin that he most likely has lymphoma. They will need to get a sample of the tumor even though it is dangerously close to his aorta.
House is discussing the rat's infection with Wilson when he notices that Kalvin's father is still sweating. House has Wilson cancel Kalvin's biopsy because it isn't the problem. House brings the father and son together to ask if they used to hunt foxes together. He then explains that the masses in Kalvin's chest aren't tumors but parasitic cysts from a bug called echinococcosis, which comes from touching a dead fox.
House asks Kalvin's father to take a blood test to determine whether they have the same symptoms. The dad chafes at the idea, so House goads the man about his dead wife until he punches House. This allows House to hit him in the kidney with his cane which ruptures one of the man's liver cysts. He falls into an anaphylactic shock, which is the same thing that happened to Kalvin earlier.
Kalvin and his father successfully recover from surgery. Afterwards, Cameron accuses Kalvin of lying to her about loosening up. She notes that he hasn't had a single visitor. Cameron accuses him of trying to self-destruct because of his mother's death.
House visits with Stacy about his latest assault. He tries to bait her into admitting that she's not sleeping with Mark, but Stacy realizes that his extreme confidence about their relationship must mean that he read her file. She throws him out of her office.