While in the hospital, the newborn Hartig daughter spits up and the mother is concerned because the baby hasn't eaten anything yet. The baby suffers a seizure. Later, a doctor who was in the room recounts the incident to another doctor in the OB/GYN lounge. He discusses the baby's bowel obstruction. House overhears their chat and quickly leaves.
House presents Baby Hartig and Baby Hausen to his staff. House thinks an infection is spreading throughout the hospital, but Dr. Cuddy doesn't believe it's an epidemic. House takes his doctors in the maternity ward to check the twelve newborns in the hospital. But they do find another baby in the overflow rooms with a sudden fever and similar symptoms.
House and crew discuss three- perhaps four- sick babies and the symptoms. With a spike in fever and low blood pressure, these children could be dead in a day. The group decide it's likely a bacterial infection. Since there's no time to wait for test results, House orders the treatments to be started. Each baby gets an MRI, but nothing shows up on the scan, so the doctors administer two broad-spectrum antibiotics. One of them starts causing the kidneys to shut down in two of the several babies. But which one? Stopping both antibiotics will kill both babies, but leaving them on both treatments will have the same result.
House says there's no point in guessing, so he takes Baby Hartig off the Aztreonam and Baby Chin-Lopino off the Vancomycin, fully aware that which ever child is receiving the ineffective antibiotic will die.
Dr. Cuddy and a hospital lawyer question the wisdom of changing the treatments without fully informing the parents. He argues that this experiment will save at least six more babies, so Cuddy gives him the green light, as usual. Later, the Chin-Lopino baby's health begins to worsen, with a falling heart rate and blood pressure. The doctors rush in to a code, trying to shock the baby back to life, but he dies. This proves that the Aztreonam doesn't work, so House instructs his staff to cover the rest of the babies with Vancomycin.
Chase informs the team that the Vancomycin isn't working either as the Hartig baby is getting worse. House performs an autopsy on Baby Chin-Lopino and realizes that it's a virus, and that is affecting the babies' hearts. Foreman complains that it could be any one of a thousand different viruses. However, with the amount of blood in the babies' bodies, they can only run five or six tests. So the team tries to narrow down the list of possibilities, and ends up with eight. They also take blood from the one healthy newborn in the hospital to use as a control.
The sick babies all test positive for Echo virus 11, CMV and Parvo virus B19. But the healthy baby tested positive for Echo and CMV antibodies. House realizes that these infants still have their mothers' blood and immune systems, so he orders a test on the mothers to see what they have antibodies for. Whatever the women are missing is what is killing their kids. Finally, the doctors settle on Echo virus 11. They get an experimental antiviral from Pennsylvania.
Chase and Foreman bring good news to the Hartigs. Their baby is recovering- all of the babies are recovering. That night, House is still obsessed with determining the person who was spreading the virus. He observes an elderly maternity ward volunteer coughing and wiping her nose as she pushes around a cart of baby toys and stuffed animals, and puts the last piece in place.