During a mundane day at Torchwood Three, the team decides to go get a drink, though Ianto politely stays behind. Once the team is gone, he brings a Japanese doctor named Tanizaki into the Hub and takes him to a holding cell deep below the surface. There, a partially cyber-converted woman is strapped to a conversion unit surrounded by instruments keeping her alive. Ianto introduces her to Tanizaki as Lisa, his former lover when they both worked at Torchwood One in London. She was only partially converted during the Cyberman invasion, and Ianto rigged the equipment to keep her alive in secret. Ianto hopes that Dr. Tanizaki, a cybernetics expert, can undo the conversion and make her human again. They take her to theautopsy lab and are able to let her breathe on her own, when Ianto learns that the rest of the team is on the way back to Torchwood to deal with a rogue UFO. Ianto leaves Dr. Tanizaki to take Lisa back to the cell while he prepares for the team's arrival. However, after he leaves, Lisa's Cyberman influence takes over, and she attempts to "upgrade" Dr. Tanizaki as way of repaying him for his favor, killing him.
As the rest of Torchwood deal with the UFO, there is a brief power flicker; Ianto quickly covers up by claiming that the generators have been acting up all day and offers to look into it. Returning to Lisa's cell, he finds Dr. Tanizaki's body with Lisa standing over him. Ianto tells Lisa to stay in the cell as he drags Dr. Tanizaki's body off to hide it from the team. Lisa reenters the cybernetic unit and begins to drain more power from Torchwood, which is quickly noticed by the others. Arming themselves, they head for the holding cell where the power drain is occurring, beli
eving Torchwood to be under attack. They find and recognize the cyber-conversion unit, but are attacked by Lisa, resulting in Owen being knocked out. Gwen points her gun tries to reason with Lisa, who knocks the gun out of Gwen's hand and pushes her over. Gwen lands on her back on the conversion unit. Before she can sit up Lisa straps her down in place. Jack attempts to shoot Lisa before she can start the unit up, but Ianto tackles him, allowing Lisa to escape after starting up the machine. The restrained Gwen is terrified and yells for help. Tosh manages to shut the unit down by entering lockdown, and eventually Jack manages to free Gwen, alowing her to get up. The team regroups in the Hub as the power is drained further by Lisa in preparation to recreate a Cyberman army. Jack demands that Ianto tell everything that's happened. Ianto pleads that since he cleans up after Torchwood but is never asked about his life, they should help him restore Lisa, but Jack maintains that there is no cure. Ianto asks that he try to reason with Lisa before the team attempts to attack her again.
Approaching Lisa, Ianto tries to remind her that she is still human, but she is disgusted by her partially-human appearance. She offers to implant her brain into Ianto, believing that sharing the same life is true love, but Ianto refuses. Lisa declares him incompatible and throws him aside, knocking Ianto unconscious, and sending the rest of the team into action. Jack orders Toshiko to go to the surface to recharge emergency power cells while Gwen and Owen are to search for any weapons they can find, while he tries to delay Lisa's advance. Lisa electrocutes Jack twice, to the shock of Gwen and Owen. Gwen and Owen are momentarily trapped by Lisa in the autopsy lab
but manage to escape back to the main Hub, where they find Jack, very much alive despite the previous attack. Jack sprays Lisa with a special "barbecue sauce" that helps their pet pterodactyl identify its prey, and as it attacks Lisa, the rest of the team, along with the recovered Ianto, are able to escape through the invisible lift. Upon exiting the lift, Ianto immediately punches Jack and tries to go back inside, but Jack threatens him, warning that if he fails to kill Lisa within ten minutes, he will come down and kill them both. Ianto returns to Torchwood by himself.
Unbeknownst to the team, a pizza delivery girl, using Torchwood's other entrance to bring some pizzas ordered earlier by Ianto, finds the pterodactyl barely on the floor, and then Lisa is suddenly behind her. As Ianto makes his way to the holding cell, he finds that Lisa's body has died. The delivery girl, with a large cut across her forehead, claims that she is Lisa, and that she took the girl's body so they could be together. Ianto hugs her, then pushes her away and holds her at gunpoint. Lisa tries to explain she did this for Ianto, but then promises that they can be upgraded together. The rest of the Torchwood team, arriving in time to hear that, open fire on the girl and kill her, leaving Ianto to mourn over Lisa's body.
The next day, Ianto recovered from sadness enters the Hub. Jack & Gwen watch Ianto cleaning up and they discuss about how Ianto couldn't bear to live without Lisa. Gwen asks Jack if he would have shot her if had stood by Ianto but Jack only replies that she had not. He refuses to tell Gwen that he loved anyone. They continue to watch.