The Enterprise arrives at Altrea IV, a planet with a molten core that is solidifying. This will eventually end all life on Altrea if unchecked. Pran and Juliana Tainer, scientists from Altrea, tell the Enterprise crew about the problem. Geordi and Data suggest injecting liquid plasma into the core, re-liquefying the core for centuries. The Enterprise will use its phasers to drill through the crust of the planet, then set up plasma infusion units just above the core.
The crew leave to implement the plan, but Juliana stops Data and asks if he recognizes her. When he doesn't, it confirms to her that all his early memories were wiped. She claims to be the former wife of Data's creator, Noonien Soong. Data can only find one Juliana in his memory, a Juliana O'Donnell. She explains that protests from her mother caused them to elope and get married. A Klingon and a Carvalan Freighter captain were their witnesses. Data investigates the ship she claims they were on and finds that they were both passengers on the ship in question. Geordi convinces Data to trust her, and he opens up to her.
She explains to Data that his 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 and forced them to flee without him. When she describes one of Noonien's qualities and he agrees, she wonders how he knows. She is delighted to learn that Data has met his father, and equally dismayed to learn that it was right before his death.
Data is troubled that Juliana did not attempt to contact him if she is his mother. Counselor Troi encourages Data to confront Juliana about the issue. Data and his mother begin to connect more. As they start the plasma infusion, Juliana informs Data and Geordi about Data's trouble with keeping his clothes on; since Data is not affected by the elements, they had to write a modesty program. They run into a problem when the phaser beam hits some magnacite ore, but Juliana is able to adjust the frequency of the beam, and much faster than anyone thought it would take. She claims to just have been lucky. While they wait for the drilled pockets to cool, Data takes his mother back to his quarters, where he plays a piece of music that he'll be playing at a later concert. She offers to play the piece with him and he replicates her a viola. She spots some of his paintings and he shows them to his mother, including one of his daughter, Lal. When Juliana learns of Lal's life and death, she once again becomes troubled.
The two practice the piece in Ten Forward, and it is clear that Juliana is upset afterwards. She asks Data if he plans to create another android child. When he indicates that he may, she has to admit that she was against Data being created due to the problems with Lore. At this point, she feels so much guilt that she must admit that there was room in her escape pod to bring Data, but she objected. She was so afraid that he would turn out like Lore, that she purposefully left him on the surface. She runs crying from Ten Forward.
The next day (presumably, since Juliana is not as upset anymore), Data, Juliana, and her husband Pran go down to Altrea to set up the plasma infusion units. Pran senses Juliana's tension around Data and mistakenly presumes Data is upset about what Juliana told him. Data explains he is incapable of the offense Pran is assigning him, but he is curious. Would Juliana have left him on Omicron Theta if he were her biological child? She bristles at the question, but must admit she would have brought him if he were human. When he presumes this means she values biological life over technological life, she is deeply offended. She loved every android they created, even Lore, and was only afraid that if Data turned out to be evil they'd have to dismantle him the way they did Lore. Back on board the Enterprise, Data and Juliana have their concert. Data observes something odd about Juliana, and asks Dr. Crusher to examine her medical records and transporter trace. Except for some high blood pressure common in women her age, nothing is wrong. Data is unconvinced and is about to look over the full records when Riker calls. One of the pockets has collapsed and he is needed in Transporter Room 2.
The two go to the Transporter Room and find Pran injured from the cave-in. The infusion unit was knocked down and must be reset, so Juliana and Data go down to fix it. The Enterprise reports to them that the pocket is currently stable but will eventually shake even worse. They cannot wait to get the entire system setup; this unit must be activated immediately, before it is damaged. They finish the job and return to the transport point, but find the pattern enhancers 30 feet down a cliff. There are no hand holds in the rock, so they must jump to safety. Data insists they can survive the jump, but Juliana doesn't think so. When Data jumps, he basically drags Juliana over the cliff. He lands safely but she falls hard. She is knocked unconscious and her right arm is completely separated. When Data looks at it, he sees a network of circuitry. Juliana Tainer is an android.
Geordi and Dr. Crusher examine her and are fascinated. She has sweat glands and tear ducts. She ages not only in appearance, but has a feedback loop processor that sends out a false human bio-signal that ages as well. Data is not surprised, however. He's already observed qualities that suggested she is an android. She performed math equations, such as finding the right frequency for the phaser beam when it hit the ore, very quickly. Later he observed that her blinking uses the exact same Fourier equations to simulate randomness that his does, and her performance in the concert was an exact note-for-note replication of her earlier practice.
In her positronic brain, Geordi finds an information chip with a holographic interface. Data activates the chip in the holodeck and sees his father. Dr. Soong created a program to answer the questions of whoever finds out Juliana is an android. When Data reveals who he is, it activates a specific subroutine for his questions. Soong explains that his wife once was a real human, but died after the Crystalline Entity attacked. He created a new android and used synaptic scanning to put Juliana's memories into it. She awoke believing she was human, and the two were happy. Until she became dissatisfied with their marriage, believing Soong loved his androids more than her, and she left him. Data starts to say that she could be happy when she finds out she is an android, but Soong cuts him off. Juliana is programmed to totally shut down in the event that she learns what she is. Soong pleads with Data to let her have her humanity, since she is programmed to die after a long life.
Data is conflicted; part of him wants to do as his father said and keep his mother happy, but he also wants to share being an android with his mother. Crusher and Picard advocate that if she should find out she's an android, she should learn that from her son. Troi objects, feeling that learning she is an android after all this time would be too shocking. She tells Data that, in a way, he would be taking from her what he has always wanted; being human.
Data returns to Sickbay and replaces the chip. When he closes Juliana's head, she awakens and smiles, recognizing Data. He tells her that she fell from the cliff and broke her arm. Dr. Crusher has repaired it, and everything is fine. As Juliana prepares to leave the ship, Data tells her "My father once told me that he had only one great love in his life. And that he regretted never telling her how much he loved her. I do believe he was referring to you." Juliana is touched, and Data agrees to visit Altrea on his next shore leave. She beams off the ship, happy and looking forward to seeing her son again.