Stardate 42073.1 : the USS Enterprise picks up Dr. Katherine Pulaski, the Enterprise's new chief medical officer (Dr. Beverly Crusher has been reassigned to Starfleet Medical). Meanwhile, Lieutenant Geordi La Forge, newly appointed Chief Engineer, is showing off his design for a specimen container, to be inserted into a large bio-containment module.
The Enterprise sets course for Aucdet IX to hand over dangerous plasma plague specimens, for transfer to the Federation medical station at Tango Sierra in the Rachelis system to help study a plague outbreak. As the ship gets underway, a ball of energy passes through the hull and explores the ship, finally drifting into Counselor Troi's quarters. While she sleeps, the ball of light slips under her blanket.
Jean-Luc Picard, informed that Dr. Pulaski is in Ten Forward, finds her there with Troi who has something important to tell him. In an urgently arranged meeting, Dr. Pulaski announces to Picard, Riker, Data and Worf that Troi is pregnant; that the conception occurred about eleven hours earlier, but on her first examination the fetus was already six weeks old; that at this rate, Troi would give birth within the next 36 hours. A stunned Riker asks who the father is. Troi says while she slept she felt a presence enter her body - she then announces firmly that she will have the baby.
The Enterprise finally arrives at Aucdet IX where Lt. Commander Hester Dealt comes aboard.
Troi begins labor; Pulaski needs help with the delivery, and Troi suggests Data, who is intrigued, saying that he understands the technical aspects of life's creation but not the process. When the child is born, Troi names him Ian Andrew after her father. Later, Picard and Pulaski visit Troi's quarters, and find that Ian now appears to be a child of four years, and already talking.
Pulaski has an issue with Data, who corrects her on the pronunciation of his name. She cannot understand why this should matter to a machine, and it is the first of several occasions in the series where she tries to belittle him as a "mere android".
Pulaski and Picard pay Troi and Ian another visit. Ian burns himself, yelps with pain, then comments that his face is wet (from tears), a new sensation for him. Pulaski suggests that Ian has burned himself on purpose. Picard agrees, "For the experience", and asks Ian directly, "Why are you here?". A confused Ian doesn't have a direct reply.
The viral samples being now aboard, the Enterprise gets underway. Down in the cargo bay, Dealt is alarmed by readings from the containment module, indicating growth in one of the samples. Geordi suggests the ship's warp field is somehow responsible. Pulaski and Riker leave the bridge to assist. Pulaski wonders why this one strain of virus is growing, and asks Dealt about its etiology; he says the sample is stimulated by Eichner radiation and scans for it, getting a positive reading. Pulaski asks what would emit such energy. Data suggests a few things but none are aboard.
Back in Troi's quarters, Ian seems upset. He senses he is the cause of worry on the ship and adds that he has to leave or things will get really bad for everyone. Troi panics, sensing the boy is dying. She calls for Dr. Pulaski, who rushes in with Picard, Riker and Data behind her and scans Ian. His life signs are fading rapidly. Data's own scans indicate the boy is the source of the radiation. Pulaski says there is nothing she can do. Troi weeps as Ian takes his final breath, but soon the boy's body begins to glow and dematerialize. The tiny mote of light that entered the ship earlier floats above the bed. Everyone looks on in wonder as the light enters Troi's open hands. She seems to communicate with the light, which then drifts away through the bulkhead and out into space.
The viral growth stops. Troi explains that Ian was a life-force entity. He found the Enterprise and became curious to know more about the crew. He decided the best way was to go through the process of being born, in order to live as one of them. She states he never meant any harm and that she will miss him.
The Enterprise finally reaches the Rachelis system. Wesley states he wishes to remain on the Enterprise, feeling this is where he belongs, although he will miss his mother greatly. Picard asks if had discussed the matter with her; he admits he wanted the Captain's permission first. The Captain agrees after some ribbing from crew on the bridge, and the ship gets underway.