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. The Enterprise taps into the Lantree 's viewer, and finds that the crew appear to have 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 in one of the ship's officers. They discover that the Lantree 's last port-of-call was the Darwin Genetic Research Station on Gagarin IV. Captain Picard orders a quarantine warning broadcast to be set on the Lantree, 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 station's genetically engineered children brought aboard, asserting that they are safe, 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 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 sick bay, 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 away from the Enterprise, so that she can examine the child outside of the styrolite without exposing the rest of the crew, and recruits Commander Data (Brent Spiner) as pilot, who as an android would be immune to possible infection. When she removes the protective material, the boy awakens and telepathically communicates with her. She is suddenly struck by joint pain, and recognizes that she has become affected with the same condition as the Lantree and station crew, and orders Data to pilot the shuttle to the research station.
As the Enterprise crew investigates using the transporter to reverse the aging process, Dr. Pulaski continues to research the cause of the syndrome at the station. She learns that the children have been genetically engineered with highly aggressive immune systems that fight pathogens by genetically altering them – not just in the children's bodies, but in the surrounding environment as well. Recalling the Lantree officer with Thelusian flu, Dr. Pulaski postulates that the children's immune systems' 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 an infected individual to normal, but only with a previous bio-pattern of that individual, and they do not have one for Dr. Pulaski. The crew finds a single hair of Pulaski's in her quarters and using it as a template is able to restore her to full health. The Enterprise uses the same procedure to restore the station's staff, noting that the children will never be able to live alongside other humans, and then returns to the contaminated Lantree to destroy the ill-fated vessel.