The U.S.S. Enterprise travels to the planet Atrea IV in order to avert an impending natural disaster. Husband-and-wife scientists Pran and Juliana Tainer explain that Atrea's molten core is solidifying, a condition that will eventually render their planet uninhabitable. Data suggests re-liquefying the core by injecting plasma directly into the planet's center, and Pran and Juliana agree to try. After the meeting, Juliana approaches Data with some surprising news — she was once married to Dr. Soong and is, in effect, his mother. She explains that Data doesn't remember her because his memory was erased shortly before the Crystalline entity's attack on Omicron Theta. Soong intended to reactivate Data, but he and Juliana were forced to flee without him. Data breaks the news that Dr. Soong is dead, and Juliana is clearly shaken. She tells Data she hopes to spend time with him, but he is disturbed by the fact that he has no memory of her.
Later, Data confides to Troi that he is confused as to how his own mother could never attempt to contact him, and Troi encourages him to confront Juliana. The next day, Juliana fills Data in on his childhood while they prepare to fire the first phaser blast to enter the magma pockets. The process goes smoothly, and Data takes Juliana back to his quarters, where he plays the violin for her. She is impressed with his talent, and is also interested in his paintings, particularly one of his late daughter, Lal. When he tells Juliana that he hopes to have another child, she admits that she was against her husband creating Data because of the way Lore turned out. She also reveals the painful truth that she and Soong left him on Omicron Theta not because they had to, but because she feared that If he were reactivated, he would become evil, like Lore.
Data, Pran, and Juliana travel to Atrea to set up the plasma infusion units in the magma pockets. Pran notices the tension between Data and his wife, and Data asks Juliana if she would have left him behind if he were her biological child. Juliana explains that while she loved him as if she gave birth to him, she could not bear the thought of being forced to dismantle him like she was with Lore. Data accepts this and relaxes, but almost immediately afterward, begins to sense something "strange" about his mother. He begins to check her medical records but is summoned to Transporter Room Two — one of the magma pockets on Atrea has caved-in.
Since Pran Is injured in the cave-in, Juliana returns with Data to finish the work before the pocket collapses. They are able to complete the job, but a series of severe tremors changes the landscape and renders their beam-out point unreachable. They must jump off a 30-foot cliff to reach safety, and Data insists that Juliana make the jump. She lands hard and is knocked unconscious, and her arm is ripped off, revealing that she is an android.
Geordi La Forge discovers an information chip inside Juliana's "brain," and Data inserts it into the Holodeck. Dr. Soong appears and talks with Data, telling him that Juliana the android was created after his wife, the human Juliana, was mortally wounded in the Crystalline Entity attack. Soong transferred her memories, and when she was activated, she had no idea that she was an android. Later, Troi encourages Data to let her live out her life believing she is human. Her advice touches Data and he keeps the secret, instead talking with his mother about Soong's great love for her. Juliana leaves the Enterprise happy, looking forward to her next visit with her son.