A prostitute, Mary, is leading a woman through the woods. When she stops him, he slaps her and she scratches him back before running off into the woods. He chases after her but Mary sees a glowing object ahead in the woods and hears a high-pitched noise. She runs to the object as the soldier sees it and draws his gun. The glow intensifies and he moves in. He finds Mary standing there calmly and shoots her.
The Torchwood team arrives at a construction site in Cardiff. The woman, Mary, wearing modern garb, is watching them. The workers have unearthed two skeletons: one is alien and one is a woman. Tosh confirms the skeletons date back to 1812. As they leave, Gwen flirts briefly with Owen and Tosh notices. Back at the hub, Gwen and Owen are working together and Owen accidentally kicks out the plug on Tosh's computer. The couple find it amusing and Tosh snaps at them for not acting professionally.
Later, Tosh goes to a bar where Mary approaches her and explains that she wants to talk to Tosh so a man won't hit on her. However, it soon becomes clear that Mary knows Tosh is with Torchwood and admits that she saw her at the building site that morning. She claims she's a scavenger, one of a number of Internet geeks who study Torchwood's activity via the Internet. Tosh is initially worried but ends up talking and drinking with her, complaining no one at Torchwood understands her. Mary offers her a pendant and when Tosh put it on, she can suddenly hear the thoughts of everyone in the bar. Overwhelmed, she asks what's happened and Mary tells her to focus and shut everything else out. Tosh says she has to show the others but Mary figures she won't.
The next day, Tosh arrives at work and puts the pendant on. She overhears both Gwen and Owen thinking about their relationship and quickly decides not to tell them about the pendant. Ianto spends most of his time thinking about the constant pain he's in. Tosh removes the pendant and goes back to her apartment. Mary is there and Tosh admits she didn't tell her teammates about the pendant. Mary tells her that much of what the amulet reveals is what people don't even realize they're thinking. She puts the amulet on Tosh and telepathically passes on her own thoughts of the two of them together. They end up in bed and afterward talk. Mary notices that Tosh has a photo of her and Owen together and wonders if they have a thing. Tosh, angry at what she's learned about Owen, turns away. However, Mary tells her that some good can come of the pendant and tells her to go out into a crowd. Tosh demands answers and Mary says she's also known as Philoctetes.
Tosh goes out onto the street and puts on the pendant. Among the burst of thoughts she hears one man thinking about murder. The man, Neil, goes home to his wife and child and pulls out a shotgun. However, Tosh arrives and knocks him out just in time.
At the Hub, Jack, Gwen, and Owen are playing around over the woman's skeleton as Tosh arrives. They explain that the skeleton is actually that of a young man and he died of an unidentified trauma, not a gunshot wound. Tosh asks Jack about Philoctetes and he notes the Greek was an ancient archer who fought in the Trojan War and was marooned on an island for 10 years.
Tosh meets with Mary and admits that she thinks the pendant is useful. Mary flirts with her again and they kiss. She then asks Tosh about what she found at the construction site. When Tosh admits Jack is handling the investigation, Mary intimates that Jack might be keeping something from her. Back at Torchwood, Tosh visits Owen, who is reviewing the corpse again. She asks him about any alien hardware that was found with the body and then puts on the pendant. However, Owen is spending most of his time thinking about Gwen as she comes in. Gwen is thinking about Owen. Disgusted, Tosh goes upstairs and sees a piece of alien equipment from the construction site. Jack arrives and tells Tosh he's heard about her stopping the attempted murder. He's curious how he overheard Danny talking about his plans to murder his family, and finds it odd that Danny would have done it on the streets as Tosh claims. Tosh redirects the conversation back to the alien hardware but he doesn’t discuss anything. When she uses the amulet, she discovers that she's unable to read Jack's thoughts. However, Jack seems to notice something.
Back at her apartment, Tosh tells Mary she's going to give her teammates the pendant. Mary says that Tosh's friends despise her and is worried that they might consider her an alien. When Tosh insists, Mary concentrates and reverts to her alien form, but says she's the same person. Tosh is shocked and Mary explains that her race uses the pendant as a form of communication. The alien hardware is a transporter that can get her home and she needs to use it before Torchwood dismantles it. Mary refuses to turn herself over to Torchwood, figuring they'll capture and interrogate her rather than try to understand her. Tosh walks the streets to think about it and is increasingly overwhelmed by all the thoughts around her. She asks Mary what to do and Mary tells her to get her into Torchwood.
Owen tries to work out how the skeleton's owner died and realizes there's been a string of similar deaths over the centuries. In each case, the victim's rib cage was shattered and the heart removed. He calls Jack.
Tosh takes Mary into Torchwood but discovers the transporter is gone. Jack arrives, holding the transporter, and says it was clear something was different about Tosh. He explains that the transporter is a two-man device. Mary admits that she arrived on Earth, killed her guard, and then possessed the prostitute Mary's body. As the others come in, Mary explains she killed the soldier and fed on his heart. She's been doing it ever since.
As Mary talks, Tosh puts on the pendant. Mary talks of how the transporter started deteriorating once it was exposed at the construction site. She then grabs Tosh and threatens to kill her if they don't turn over the transporter. When she offers to trade Tosh for Gwen as a hostage, Tosh hears Owen balk mentally and Mary points out that she's the only one who cares about Tosh. Jack sends his own thoughts, telling Tosh to play along with him. He offers the transporter for Tosh and Mary releases her. However, when Mary takes the transporter, Jack reveals that he reprogrammed it. Mary disappears in a flash of light as Jack explains to a shocked Tosh that he set the coordinates to the center of the Sun.
Later, Gwen and Owen ask Tosh what she overheard. Tosh tries to defer and Owen points out that what she heard was none of their business. Owen stalks off and Tosh tries to apologize to Gwen. Gwen admits that it's a wake-up call about her own betrayal of her fiancé, but she won't, and admits she's in no position to make judgments either. Outside, Tosh talks to Jack and says it's the most powerful piece of technology they've ever had. He lets her decide what to do with it and she crushes it. She asks why she couldn't read his mind and Jack admits he could feel her in there. Tosh notes that it seemed like he was dead. Finally she wonders how she can live with the knowledge she's gained, not just about her teammates but about the entire world. Jack simply turns and walks away.