House and Stacy are out to dinner, waiting for the arrival of her husband, Mark. Once again, he is late because he's been avoiding House. When Mark finally arrives, he announces that he has already consulted other doctors because he didn't want to waste House's valuable time. The doctors thought he was suffering from stress, so Mark declares himself to be fine.
Mark and House politely bicker with their rivalry over Stacy coming to the surface. The argument becomes more competitive as the two men race to down their beers. Mark suddenly becomes woozy and passes out. House drugged him in order to get him back to the hospital for a checkup.
Cameron and Foreman review Mark's file. He has experienced stomach pains and mood swings with no apparent source. Tests have shown nothing. House, making no effort to hide his disdain for Mark, orders another battery of tests. Out in the hallway, Wilson reminds House of his past with Stacy and advises him to treat this case like any other patient -- by being cold and distant.
Cameron explains to Mark that a CT of his abdomen revealed nothing. He prepares to leave. Foreman tells House that the surgeon is booked. Cameron can't believe that House would plan exploratory surgery for a patient who doesn't think he's sick.
House joins Stacy in the waiting room. The surgery reveals nothing but a distended bladder. House asks for video of the surgery. Wilson interrupts to say he saw House talking to Stacy. Wasn't he supposed to stay away from her? House admits that he likes Stacy -- which is why he was talking to her -- but he can handle everything.
House continues poring over the video. He notices something and rouses his team from their sleep to come in. They don't see anything. House shows them a comparison video and they immediately spot the problem. There are tremors in the muscle fiber, which is an abdominal epilepsy. There must be some sort of serious neurological problem.
Later that morning, Foreman brings in Mark's EEG graph. He's noticed a small abnormality in the brain waves, which means his axonal nerves are dying. The likely causes are encephalitis or Alzheimer's. Cameron points out that Early Onset Alzheimer's is the worst kind. House asks for viral serologies to rule out encephalitis and Tau proteins to check for Alzheimer's. He also asks the team to check out Mark and Stacy's house for possible indicators.
Cameron is in the lab when Stacy demands to know what their team is thinking. Cameron admits that Alzheimer's is the early guess. Cameron then asks Stacy what House was like before his leg troubles. She says that he was pretty much the same as he is now. The lab test finishes as Stacy is waiting. Mark tests negatively for Alzheimer's.
Chase and Foreman are out at the house and find an unused mountain bike and a yoga mat. Mark has recently changed his workout. Chase finds a bottle of amphetamines hidden in the back of a desk drawer underneath some papers. They bring it back to House.
Although regular use could explain the symptoms, House doesn't agree. The prescribing doctor recently had his license pulled for writing illegal prescriptions to high school kids. Since Mark is a guidance counselor, it is more likely that he confiscated the pills from a student.
With no trace of encephalitis or Alzheimer's, House orders a PET scan to check for alteration in the metabolic activity of the cerebral cortex.
Foreman injects a chemical marker called FDDNP into Mark that will highlight brain functions. Mark slides into the PET scanner and Foreman begins to ask him simple questions about his life. House interrupts and begins to quiz Mark about his wedding day. Mark angrily replies that Stacy wore a white dress and that they didn't leave their Paris hotel room for the entire two weeks of their honeymoon.
That night, House stands on the roof. Stacy barges outside, angry that House quizzed Mark about her. House explains that he was testing the operational parameters of Mark's limbic system, but Stacy isn't buying this claim.
House admits that he doesn't know what's wrong with Mark. Everything is coming up negative. Mark is perfectly healthy, but his brain is dying. Stacy is devastated that House can't figure it out. He assures her that he hasn't given up. They just have to wait for something to change.
That night, a frantic Mark calls in the nurses and Stacy. He can't feel his feet or his hands. He's paralyzed. House and the team retreat to his office to find explanations. Realizing that the paralysis is peripheral, House suggests Guillain-Barre syndrome. Foreman says he's done the tests already and no antibodies have shown up.
House wonders if Mark has the virus but isn't producing antibodies. Foreman realizes that would mean Mark is sick, but his body isn't doing anything about it. They will have to do it for him. Yet without antibodies, they'll never know if they are correct. House thinks that if the treatment works, they were right. If Mark dies, it was something else.
Cameron explains to Mark what they're going to try. Mark is hooked up to a plasmapheresis machine which will clean his blood. Mark then notices House call Stacy out into the hallway. Mark becomes paranoid that Stacy is going to leave him. He struggles to breathe. House enters the room and orders two milligrams of Ativan to calm Mark down. It wasn't an allergic reaction, just a panic attack.
Out in the hallway, House asks Stacy about the cards that she and Mark have been receiving. He doesn't notice any of her old friends on them. He also finds it odd that she didn't tell her old friend Dr. Wilson that she would be in the area. Stacy wants to know what his point is. Is she still in love with him? Should she abandon her dying husband for him? Stacy thinks maybe Wilson is right about House staying away from her.
House drowns his sorrows at a bar when Wilson comes in. Although House had called him there, Wilson is in a hurry because he was in the middle of his wife's dinner party. House tells him that Mark isn't responding to treatment and he is happy about that. However, House isn't sure whether it's because he hopes to reunite with Stacy or that he merely wants her to suffer. Wilson has no response.
The next morning, House again calls Stacy out from Mark's room for a talk. House confides that he thinks she and Mark are good together, especially because he took her to Paris, her dream city. Stacy says that they never went to Paris on their honeymoon. Since she had to work, they spent a couple of nights in New York.
House is intrigued, and asks her when Mark switched from mountain biking to yoga. She responds that it was about a month ago and House rushes off. Bursting into his office, House puts up Mark's PET scan x-ray for the team to see.
Lying is a very creative process which is more involved than telling the truth. What does the PET scan show when House asked Mark about the honeymoon? The brain shows minimal involvement. They never went, but Mark's brain thinks they did.
House sums up the symptoms again: abdominal pain, peripheral neuropathy, paranoia and now delusions. House thinks Mark suffers from Acute Intermittent Porphyria, which doesn't show in tests, only when caught in the middle of an attack. Another symptom is light sensitivity, which might make a person give up mountain biking for an indoor sport like yoga.
House orders his doctors to start Mark on Hematin and glucose, but Cameron points out that if he doesn't have AIP, the Hematin will give Mark renal failure. The only way to confirm AIP is to sample urine during an attack. But how will they know an attack is coming? AIP has very specific triggers, such as barbiturates, alcohol and high protein levels. House is going to give Mark all of these and wait to see what happens. Stacy asks for a minute with Mark.
Stacy tells House that Mark doesn't want the shot. She asks House to give it to him anyway, but House refuses. This kicks off a tremendous argument about their past. Stacy thinks that House wants Mark to die so they can get back together. House says she should force him into the treatment because she's good at that. She demands to know if this is payback for his leg. She thinks she saved his life. House claims that he's respecting Mark's decision, which Stacy calls him on. He constantly browbeats patients into accepting treatment. The only difference is that Stacy had done it to him.
Mark still refuses the trigger. Yet since he is paralyzed, House is about to inject the syringe into the IV. Foreman takes the line from House's hand. Cameron says he needs a court order to go around Mark. Even Chase steps in.
House begins to walk away, then quickly turns back and stabs Mark in the thigh. For a moment, nothing happens. Suddenly, Mark's body begins to spasm. House orders Chase to draw urine from the catheter. House sticks a needle into Mark's bladder to pull a sample, then allows Cameron to inject Mark with Ativan, which calms him down.
In the lab, the team examines Mark's urine. It is still yellow, which means there is no AIP. House shines a light on the test tube to stimulate oxidation. Suddenly, the urine in test tube turns black. House tells his team to start Mark on glucose and Hematin.
Eventually, Mark can move his extremities again. The treatment is a success. That night, Stacy finds House in his office. She admits that she isn't over him and that he was always the one for her. But she can't be with him because, when they were together, she was lonely. Mark makes room in his life for her.
The next day, Cuddy intercepts House in the hallway with some news. Since Stacy's husband is going to need monitoring for some time, and they could definitely use her again, Cuddy offered her a consulting job. But Stacy will only accept if it's OK with House. He agrees.