According to Troi, a glowing white light impregnated her while she was asleep. Dr. Katherine Pulaski, the ship's new Chief Medical Officer, confirms Troi's prognosis and reports that the fetus will reach full term within 36 hours. Not knowing the true origins or intent of this mysterious child, Picard puts the crew on security alert. But when Troi gives birth to a boy named Ian, he appears to be completely harmless — although he amazes his mother and the crew when he grows from an infant to an eight-year-old in one day.

Meanwhile, Wesley Crusher reluctantly prepares to leave the U.S.S. Enterprise and join his mother, who has departed to become head of Starfleet Medical. Full of mixed emotions, Wesley decides to stay with the ship after receiving encouragement from a new crew member named Guinan. Later, when Wesley discusses his plans with Picard, the Captain agrees to let the young ensign remain as long as his mother permits it.

However, a sudden crisis with the plasma plague puts everyone's future in doubt. It seems that a contaminated specimen of the hazardous strain has inexplicably begun to grow on the cargo deck due to radiation from an unknown source on board. Dr. Pulaski reports that, at its current growth rate, the plague could wipe out the ship's entire population within two hours.

Unknown to all, Ian is the source of the radiation. When the young boy realizes this he decides to sacrifice his life to save the crew.

As a heartbroken Troi looks on, Ian dies and his body reverts to its original state — a glowing white life-force entity, which tells Troi that Ian had come to her to learn more about humans by experiencing life among them. Once the entity leaves the ship, the threat is averted and the crew continues their humanitarian mission.