Stardate 42303.7: the USS Enterprise is en route to Star Station India when a gasping distress call from the USS Lantree is received. Captain Picard orders a course change and they find the Lantree adrift with no life signs, but systems functioning normally. A visual of the bridge shows the Lantree crew as shriveled corpses.
Doctor Pulaski reports they all died from "natural causes" - old age. Yet its captain was the same age as Commander Riker. The ship's medical logs show all hands in perfect health apart from the second officer having Thelusian Flu; Pulaski adds the flu is a harmless virus. Riker says that the Lantree's last port of call was the Darwin Genetic Research Station at Gagarin IV. Picard orders course for Gagarin, and places a Starfleet medical quarantine on the Lantree.
Picard contacts Gagarin chief scientist Dr. Sara Kingsley, (a woman already aging) who reports a medical emergency - the rapid onset of geriatric phenomena - which she believes was caused by contact with the Lantree three days before. Kingsley is concerned for "the children", saying they have 32 in isolation and asking they be evacuated to the Enterprise. Picard won't put his crew at risk by transporting anyone up unless there is proof they haven't been exposed. Pulaski suggests they beam a child aboard encased in styrolite under suspended animation, so there is no risk of contamination - then she can examine them. Picard reluctantly approves.
Lieutenant La Forge readies a force field around the examination table in sickbay. A "child" is beamed up : a young male, but barely a child, encased in a membrane. When Geordi lowers the force field, Deanna Troi senses a telepathic presence in the room coming from the boy. Pulaski scans him: he's in perfect health but very advanced. She adds "We may be seeing the next step in human evolution", and asks that he be removed from the styrolite. Picard refuses, the risk is too great. Pulaski suggests she can make her examination in a shuttle craft, a secure environment, without risk to the ship and that Data can pilot the shuttle since he'll be immune to infection. Picard agrees.
Chief O'Brien transports the boy to the shuttle. Pulaski removes the styrolite and the boy awakens. Pulaski is stunned as the boy telepathically tells her his name is David. Shortly after, Pulaski feels intense joint pain in her arm, and a scan indicates the onset of the rapid aging condition. Picard orders Pulaski to return to the Enterprise, but Pulaski says she could infect the crew and says she must go to the research station.
Picard asks for ideas how to save Pulaski and Data. Geordi says the transporter's bio-filter can purge Data of contamination. The same would be true for Pulaski, but physical effects of the rapid aging can't be removed without Pulaski's original "trace" - her genetic pattern, kept in the transporter's pattern buffer after beam out. Riker says Pulaski has never used the transporter; she originally came aboard via shuttle from the USS Repulse. Picard wonders if the Repulse would have her original pattern.
Pulaski, (already showing crow's feet around the eyes) and Data arrive at the station. Kingsley is bewildered that the children carry the contagion; they have enhanced immune systems to ward off any disease; the children were genetically engineered to be perfect. Their immune systems destroy invading organisms by genetically altering them. Pulaski suddenly remembers the "Thelusian Flu" aboard the Lantree and asks how the children's immune systems would react to it. Kingsley says it would be genetically altered to extinction. Pulaski believes that's what happening â the children's immune systems are attacking everyone.
Back on the Enterprise, Picard talks with Captain Taggert of the Repulse who tells Picard that Pulaski's pattern was unfortunately deleted soon after her transfer.
Data returns to the ship as the bio-filter screens out the enhanced antibodies. Picard grieves at not being able to save the Doctor. Troi tells him not to be so hard on himself saying; "Kate knew what she was doing." Suddenly, Picard has an insight and believes he knows a solution. He asks if Data was immune because he has no biological tissue, and if the alteration of Pulaski's DNA caused her rapid aging. Data confirms this. Picard says he has one more card to play. He rushes to Geordi and says if they had Pulaski's original DNA pattern they could repair the damage caused by the aging. Geordi confirms, but says they don't have the pattern. Data and Riker dig through sickbay looking for a DNA sample, but can't find one. Then the two head for Pulaski's quarters where they find a single hair on one of her brushes.
Picard calls Pulaski. He tells her they hope to use a sample of her unaltered DNA as a filter to screen out any changes in her genome, to restore her to her proper age. Geordi and Data rush to get the DNA sequence into the transporter buffer while Pulaski nervously stands by. With everything set, Picard says he'll operate the transporter himself. Pulaski begins to fade into existence - her form coalescing into shape for several tense moments. When the beams fade, Pulaski appears with all signs of aging gone. Picard welcomes her back aboard and they hug. The scientists at the genetic station begin to restore themselves as well.
The Enterprise then heads back to the quarantined USS Lantree waits. Picard orders a torpedo to destroy the ill-fated vessel.