Plot summary
The new Chief Medical Officer, Dr. Katherine Pulaski (Diana Muldaur), is brought aboard the Enterprise as it prepares to travel to Aucdet IX in order to take on dangerous virus samples, which will be carried to a Starfleet Medical station, hoping they will be able to devise a cure for a plague epidemic in the Rachelis System. After taking aboard the new doctor, the ship is traveling at sub-light speed when a ball of energy passes in through the hull, eventually settling in the womb of Counselor Deanna Troi (Marina Sirtis).
Captain Jean-Luc Picard (Patrick Stewart) wonders why the new doctor hadn't checked in with him, and locates her in the Ten Forward lounge, where the bartender, Guinan (Whoopi Goldberg), works. Picard finds Doctor Pulaski talking with Troi about her unexpected pregnancy. The senior officers meet to discuss the pregnancy. The fetus is developing at an accelerated rate and would be fully developed in 36 hours. Troi does not know who the father is, but was aware of a "presence" entering her body the night before. Though the senior staff debate terminating the pregnancy, Troi decides she will carry the child to term.
The Enterprise arrives at Aucdet IX to pick up the plague samples, which are stored in a highly secure storage vessel. Troi gives birth to an apparently normal boy, whom she names Ian Andrew after her father. Ian continues to develop rapidly; within a day, he appears as a four-year-old child with corresponding mental faculties. When asked if he is ready to explain who or what he is, Ian responds "not yet." Having completed the transfer of the virus samples, the Enterprise heads for the Starfleet Medical station. En route, the crew finds one of the plague strains is growing inexplicably; should it continue growing, it will rupture the storage vessel and result in the catastrophic exposure of all on board.
They discover that an unknown source of Eichner radiation is causing the growth. Ian (R. J. Williams), now appearing as an eight-year-old, confides to Troi that he is the source of the crew's problems and will have to leave. Troi realizes Ian is dying and calls for medical assistance. Ian dies in Troi's arms and returns to his energy form. The energy being contacts Troi's mind, and explains that he impregnated her in order to discover what it was like to be human. With Ian gone, the plague sample returns to normal and the crew continues on their mission.
Meanwhile, Wesley Crusher (Wil Wheaton) has been trying to cope with leaving the Enterprise to join his mother on Earth after her promotion to director of Starfleet Medical. After receiving guidance from Guinan, he decides he wants to remain on the Enterprise.
Credits
Adapted from Wikipedia contributors, The Child (Star Trek: The Next Generation). Paragraphs and formatting adjusted; illustrations, reference markers, block quotations and non-story material omitted.