Set in the 24th century, the series follows the adventures of the crew of the Federation starship Enterprise. In this episode, the Enterprise encounters a Starfleet supply ship where everyone has died from rapid aging. They must find the cause before scientists on a research colony suffer the same fate.
The Federation starship Enterprise, under the command of Captain Jean-Luc Picard (Patrick Stewart), receives a distress call from the USS Lantree and arrives at the ship's last location to find it adrift. A visual scan of the ship shows all the crew as having died from old age, even though many were as young as the Enterprise crew. The only evidence of any medical problems was a harmless case of Thelusian flu by one of the ship's officers. Eventually, they find, from the Lantree's records of their last port-of-call, the Darwin Genetic Research Station on Gagarin IV. Captain Picard orders the Lantree's computer be set to broadcast a warning of medical quarantine and orders the Enterprise to Gagarin IV. As they near the planet, they receive another distress call from the station's researchers, all of whom have started a rapid onset of geriatric phenomena they believe is tied to the Lantree.
When the Enterprise arrives in orbit, the research team's leader pleads to have the genetically engineered children brought aboard as they have been in isolation from the rest of the station. Picard is concerned with exposing the Enterprise to the same phenomenon that is affecting the Darwin station, but allows Chief Medical Officer Dr. Pulaski (Diana Muldaur) to have one of the children beamed aboard encased in styrolite which shields the rest of the ship from possible contamination. The child is brought to Sickbay, and Dr. Pulaski finds the child, a male teenager, to be near perfection, describing him as the "next step in human evolution". She convinces Picard to allow her to take a shuttle piloted by the androidCommander Data (Brent Spiner) away from the Enterprise, so that she can examine the child outside of the styrolite without exposing the rest of the crew. When she removes the material, the boy awakens and is able to telepathically communicate with her. She is suddenly struck by joint pain, recognizes that she has the onset of the rapid aging syndrome and orders Data to pilot the shuttle to the Darwin research station rather than the Enterprise, which is still contamination-free.
As the Enterprise crew tries to determine how to reverse the aging process by use of the bio-filters on the transporter buffers, Dr. Pulaski continues to research the cause of the syndrome at the station. She learns that the children are all genetically engineered with highly advanced immune systems that would destroy invading organisms by genetically altering them. Recalling the Lantree officer with Thelusian flu, Dr. Pulaski postulates that the children's immune system reaction to the virus has created airborne antibodies that are attacking everything else. Knowing this, the Enterprise's transporters can remove the antibodies and return the affected people to normal, but would require a stored bio-pattern of the person in question; Dr. Pulaski unfortunately lacks one. The crew is able to find a single hair of Pulaski in her quarters and uses it as a template to attempt to restore her. The transport filter is successful, and Dr. Pulaski, back to her "youthful" self, is welcomed aboard the Enterprise. The Enterprise assists the rest of the Darwin station staff to return to normal, and then returns to the contaminated Lantree to destroy the ill-fated vessel.