The Enterprise arrives at Altrea IV, a planet with a core that is cooling and solidifying. Pran and Juliana Tainer, scientists from Altrea IV, brief the crew on the problem; Lieutenant Commanders Geordi La Forge (LeVar Burton) and Data suggest a solution.
After the others leave the briefing, Juliana reveals herself to be the former wife of Data's creator, Noonien Soong. She explains that Data's early memories were wiped and replaced with memories of the colonists of Omicron Theta. He was about to be reactivated when the Crystalline Entity attacked. Data conducts some research and finds evidence to circumstantially support her claims, accepting her as his mother. Data shows her his quarters, where he plays his violin. She joins in with him, playing a viola. Among his paintings, she sees one of his daughter, Lal, and is saddened to learn of Lal's demise. She admits that she was against Data being created due to the problems with Lore, and confesses that she forced Noonien to leave Data behind for that reason.
An emergency requires that Juliana and Data go down to effect repairs. When finished, an incident requires them to jump to safety. Juliana is knocked unconscious when they land, and her arm becomes detached, revealing a network of circuitry; Juliana is an android.
In Juliana's positronic brain, La Forge finds a chip with a holographic interface. Data activates the chip in the holodeck and sees his father. Soong explains that his wife was human, but was mortally wounded in the Crystalline Entity's attack. He created the android and used synaptic scanning to place Juliana's memories into it. She awoke believing she was human. She later chose to leave him, and he admits that the real Juliana would have left him too. Soong asks Data to let her have her humanity. Data remarks that he suspected Juliana was an android because she exactly matched his mathematical calculations, his musicianship, and his eye blinking pattern.
Data replaces the chip into Juliana's head. When she awakens, he tells her that Dr. Crusher repaired her broken arm, and everything is fine. As Juliana prepares to leave the ship, Data tells her "My father told me that he had only one great love in his life. I am certain he was referring to you."