S5E6 · Christmas Carol

Around Christmas time, Dana Scully (Gillian Anderson) and her mother visit her brother Bill and his wife, Tara, who is expecting. Scully answers the phone, and the person on the other end, who sounds just like her dead sister, Melissa, tells her that someone needs her help. Scully traces the call to a nearby home in San Diego, where local police are investigating the suicide of a woman, Roberta Sim. Kresge, the lead detective on the case, tells Scully that it was impossible for Roberta to have dialed, as she died before the telephone call was made.

That night, after dinner, Scully reveals to her mother that, due to her abduction and cancer, she is unable to bear children. Scully flashes back to when, as a child, she hid her pet rabbit from her brother in a lunchbox, only for it to suffocate and die.

Scully receives another telephone call from the same person, which was once again made from the Sim home. Roberta's husband, Marshall, is meeting with two men in black inside his house, and has no desire to listen to and help Scully figure out what is going on.

Scully visits Kresge, to look further into Roberta Sim's death, despite the fact that the police think it is a simple suicide. She finds a striking resemblance between the Sims' daughter, Emily, and her sister, Melissa, from when Melissa was that age. Scully flashes back to a funeral she attended when she was a little girl, but imagines Marshall Sim holding her hand.

Scully insists on performing an autopsy on Roberta, thinking that Roberta was murdered. She finds a needle puncture in Roberta's foot, causing her to believe that Roberta was anesthetized and her suicide was staged. The police search the Sim's house and find a used hypodermic needle, which Marshall claims was for daily injections for Emily's anemia. Scully spots the men in black watching from a nearby car.

Scully receives DNA test results on Melissa, and, matching it up to Emily's DNA results, finds them nearly identical, causing Scully to believe that Melissa is Emily's mother. Scully believes that Melissa gave birth to Emily while on the West Coast, then gave her up for adoption, without ever telling the rest of the family. Scully flashes back to when she and Melissa were teenagers and were given cross necklaces from their mother for Christmas.

Kresge tells Scully that the Sims received several large payments from a pharmaceutical company, Prangen Industries. The two visit Dr. Calderon, who tells them that Emily was part of clinical trials, and that Roberta was paid the money to keep her from pulling Emily from the program. Marshall Sim is arrested for the murder of his wife. Scully gives Emily her cross necklace. Marshall soon confesses, but is found dead in his cell after being visited by the two dark-suited men.

Bill shows Scully a photo of Melissa, who is clearly not pregnant, shortly before Emily was born, which he thinks proves that Melissa isn't Emily's mother.

Scully meets with someone from an adoptive agency, because she wants to adopt Emily. The woman is very hesitant, considering Scully's job and the fact that Emily is a special needs child. Scully flashes back to talking with Melissa around Christmas time, shortly before she joined the FBI.

On Christmas morning, Scully receives the results of an RFLP test that she requested from the FBI. The test results prove that Scully, not Melissa, is Emily's mother.