At the USA Swimming and Diving National Championships, 12-year-old Mary Carroll prepares to dive from 10 meters. On top of the platform, Mary's vision becomes blurry. She nervously approaches the edge, talks herself into the dive and leaps. When she surfaces, Mary finds the crowd swarming a man who collapsed next to the pool and is bleeding from his ear.
House goes to Cameron's place to tell her that Vogler is gone and he wants her back at work. House gets a page about an epidemic, but Cameron asks why he wants her back. He replies that it's because she's a good doctor. Since that's not what she wanted to hear, she slams the door on him.
House arrives at the hospital to find hundreds of people in the lobby. Cuddy reports that the man who collapsed at the meet has bacterial meningitis. Five thousand people were exposed who are all being sent to hospitals throughout the region. She needs House to pitch in. House, Wilson, Chase, Foreman and the rest of the staff begin examining patients and either discharging them or sending them to the second floor for treatment.
Mary comes through House's line. She has fever, a rash and neck pain, which are three of the symptoms. He also notices that she moves her head oddly, and she comments that it only hurts moving side to side, not up and down. Meningitis typically causes pus in the spinal canal that makes up and down movements painful. House quickly gathers his staff to discuss. Mary's rash has been present for a week. If it were meningitis, she'd already be dead. House orders a lumbar puncture and has Chase research all known causes of neck pain. He also orders rifampin for meningitis, just in case he's wrong.
Foreman can't get a bed and nurse for the lumbar puncture due to the overload. So he performs the procedure in the hallway. Chase searches online for a possible explanation. House makes it clear that this is just busy work for Chase as a form of punishment. Later, Foreman returns to check on Mary, but she's been moved because the staff needed a gurney. He finds her on a couch and tells Mary that she has no meningitis or infections. They're going to keep her overnight, but she shouldn't worry. That's when Foreman notices blood in Mary's mouth.
Still working out in the hallway, Chase and Foreman insert an endoscope into Mary's throat. The camera into her GI tract will show where the blood is coming from. It shows nothing. Next, they give her a pillcam. A tiny camera that she swallows and sends back video from the intestines. Reviewing the images, House notices a Dieulafoy on Mary's intestines. Cuddy interrupts and sends House back to clinic duty to handle the crush. While examining quarantined patients, Chase, Foreman, House and Wilson discuss possible illnesses for Mary. Chase throws out bone cancer, which Wilson agrees would explain the meningeal symptoms. House orders a marrow sample.
Foreman begs again for a sterile environment for Mary's procedure, again with no luck. He brings her down to the morgue to take the sample. After a few hours, he returns to Mary's spot in the hallway to deliver the good news. It's not cancer. Mary smiles, but then the EEG begins going crazy. Mary looks fine, but is unresponsive. She's having an absence seizure. Foreman quickly orders two milligrams of Ativan.
House, Foreman and Chase discuss the absence seizure in the bathroom, desperate to hide from Cuddy's demands to work the clinic. Foreman reports that the seizure frequency is increasing, with five in the last half hour. Whatever she has, it's in her brain. House wants a CT scan to check for intercranial bleeding, but the scanner is backed up with meningitis patients. Chase suggests an old method, transcranial ultrasound. It will at least reveal bleeding.
Meanwhile, House and Wilson interview a second candidate to replace Cameron. After rejecting the first young doctor for having an Asian letter tattoo and caring too much about what people think of him, House rejects the second. She's a young woman who has a fantastic background and a mouth to counter House's barbs. Yet her pointy-heeled shoes mean she also cares too much about what people think of her.
Foreman injects Mary with gadolinium, which will help the blood show on an x-ray. Chase continues poring through research materials and places a lock of hair into a gas chromatograph. Back in the clinic, Wilson chastises House for continually pushing people away for the tiniest flaws. Foreman reports significant bleeding in Mary's temporal lobe and Chase finds no sign of poisoning. House asks Cuddy for an operating room and a neurosurgeon. They rush Mary into surgery.
Mary's parents arrive at the hospital after the surgery. Foreman informs them that the operation went well and the swelling and pressure are going down. In his office, House peruses the board listing Mary's symptoms. Chase and Foreman run through possible explanations. What else do they know about Mary? She's twelve, she travels a lot and she's not afraid of heights. House suddenly heads for the door. Chase and Foreman follow.
The doctors look at Mary as she recovers in a room. House notices all the balloons. She's quite popular, but none are from male teammates. Is it because she's so young or are they purposefully avoiding her? House orders a recheck of Mary's red blood cells to see if they are intact. Under the microscope, Chase notices that Mary's blood looks like it's been in a blender. Foreman thinks it's purpura and she is bleeding into her skin. Yet ther is no e coli and she is not menopausal. There might be one other possible cause -- pregnancy.
House performs an ultrasound, which reveals a six-week old fetus. He wonders if a couple of sneaked-in beers led to a lost night, but Mary insists that it wasn't the guy's fault. She knew what she was doing. House is forced to perform a plasmapheresis to clean the antibodies from Mary's blood, which means they will have to terminate the pregnancy. Mary doesn't want her parents to find out. Under state law, House must obey this even though he knows keeping the secret is the wrong decision. House tells Mary's parents that their daughter has TTP, which is easily curable. They don't need to worry about what TTP stands for because it's only a bunch of big words they've never heard before. They press House for details, but he evades them by saying the hospital will perform a small surgical procedure to remove an unnatural growth.
After the procedure and the plasmapheresis, Mary is greatly improved. House spies from afar as Mary's parents visit their little girl. Mary breaks down and tells her parents everything, just what House hoped would happen. The next day, House interviews another prospective candidate, but he tells her there's no open position.
House heads back to Cameron's apartment to say that he needs her back. She claims that she's accepted another position, but House tells her to drop it. He wants her back, and offers up more money and a car allowance. Yet she only wants a dinner as a date, not as a colleague. He agrees and they shake hands over this odd deal. Cameron says she'll be back at work the next morning.