The Enterprise receives a distress call from the cargo ship U.S.S. Lantree. They ask for help, and say something about "dying", then cease responding at all. When the Enterprise finds it, they find no life signs, but no sign of battle damage. Picard accesses the Lantree's computer remotely and calls up their viewscreen, where they see evidence that the crew died of extreme old age (which is impossible, as Captain Talacca is only Riker's age).

When they download the Lantree's logs, they find that no one on board that ship knew what was going on either. Pulaski checks their medical histories: they were all fine eight weeks ago, and five days ago, the only illness on board was a mild case of Thalusian flu, which could never have done something like this. The Enterprise sets quarantine beacons on the Lantree and departs for its last port of call, the Darwin Genetic Research Station, both to find out if they know anything about what's happened, and to warn them if they do not.

At Darwin Station, they find the same process occurring, although more slowly. Furthermore, they believe the Lantree infected them. The head of the station, Dr. Kingsley, asks the Enterprise (and Dr. Pulaski in particular) to help their children, who have been in protective isolation and are harmless. Picard grudgingly allows one child to be beamed aboard encased in Styrolite, for perfect safety. When he's beamed aboard, he looks more like 22 than 12, seems in perfect health, and is deemed telepathic by Troi. When Pulaski wants to take him out of the Styrolite to continue tests, however, Picard refuses, because there's no way to isolate Sickbay. After discussing alternatives with Deanna and Geordi, Pulaski suggests a shuttlecraft, to which Picard agrees. Pulaski and Data take the child onto a shuttle, where Pulaski, to her surprise, is infected.

The child and the shuttle return to Darwin Station, where Pulaski and Data determine that although the children are not carriers, they are the source. The overactive immune system of the children responded to the Thalusian flu from the Lantree, and began affecting ordinary DNA, simulating aging-and the process, it seems, is irreversible.

Picard, however, is unwilling to accept this. The Enterprise crew seize on the idea of using the transporter trace pattern to control the reconstitution, but find Pulaski's never used the transporter. Picard suggests using a sample of her ordinary DNA, which they find on a hairbrush. Despite the chance that they might have to beam her energy out into space, they beam her back, successfully reversing the aging process. Unfortunately, however, the children must remain in isolation forever (although the researchers are saved), and the Enterprise returns to destroy the Lantree for the safety of humanity.