At St. Teilo's Military Hospital in 1918, Gerald Carter and Harriet Derbyshire are checking out a hospital using advanced equipment. They startle a nurse, who admits that she's seen three ghosts that day alone in the ward. Gerald and Harriet go there and Harriet notes that they'll be assigned back to the front as soon as they recover. The lights flicker and the two persons track the signal to a room. There's a flash of light and they look on as two people appear: Toshiko Sato and a young man, Tommy. Tosh tells the man to tell them what to do and insists he's the only one who can stop it or it's the end of everything. Tommy finally tells them that he's in the ward and he needs to be with his own self. Gerald and Harriet go to the 1918 Tommy and tell him that he has to come with them, and that they're Torchwood.
It's Friday the 20th and Tosh is preparing for work. At the Hub, Jack is briefing Gwen on Tommy Reginald Brockless, a 24 year old soldier kept in cryogenic suspension. Torchwood froze Tommy in 1918 and they have to wake him up once a day every year to check on him. Jack says that one day they'll need him. Tosh, wearing a dress, looks on as Owen revives Tommy. He revives and shoves them aside, but Tosh finally manages to calm him down. He comes out of his shock and Tosh helps him remember who she is and where he is.
Tommy eats with the team and compliments Tosh on her dress. Owen then tests his reflexes and takes blood samples. Tosh checks his memories, including the death of his parents. Meanwhile, Jack explains to Gwen that there was a time rift at St. Teilo's in 1918 and it merged with the slice of the future. However, they don't now what piece of the future it merged. Once the time rift catches up to the presents, chunks of the hospital in 1918 will start appearing at the current-day site. When the time shift is complete, it'll start a chain reaction causing time shifts all over the world. The Torchwood of 1918 left sealed orders inside a box with a temporal lock. It can't be opened until the time rift starts to manifest. Tosh brings Tommy in to inform everyone that he's okay, and the two of them head out for a date.
Tommy and Tosh go around Cardiff and Tosh brings him up to date on what she's been doing. He notes that she always says that she's working and never learns Spanish or playing the piano. Tosh admits that Torchwood takes up all of her time but Tommy wonders why when she's a volunteer.
Gwen and Ianto review the files on Gerald and Harriet and Ianto notes that Harriet and many of the other agents died young, and nothing changes. She leaves for St. Telio's.
Tosh and Tommy play pool at a pub and he asks if she has a boyfriend. She admits she hasn't and asks if he had a girlfriend in 1918. He admits he had a girl, Ellie, and they courted for two years. Then he stopped seeing her the last time he was home on leave, because the war changed him.
At the now-abandoned hospital, Gwen looks around and hears the faint echo of laughter. She comes face-to-face with a one-legged soldier from 1918, but he doesn't respond to her. He hobbles forward toward her and Gwen backs away. He walks past her and disappears, seemingly unaware of her presence. She continues to search as the electrical lights start flickering and then go out entirely. Several workers arrive, startling her.
Gwen calls in Jack to inform him that the workers have informed her they're knocking down the hospital, which could set off the time rift. They check with Owen, who is monitoring for time spikes in Gwen's absence and says there's nothing yet.
At the pub, Tommy sees a broadcast about the Iraqi War. He notes that they told him the war was over the first year they awoke him, and that three weeks later by his time, World War II began. Tommy wonders if the human race is worth saving. Tosh insists it is, and Tommy notes that he'd do anything for her. He suddenly clutches at his forehead and says he feels something.
At St. Telio's, the workers start bringing down the building. Jack hears something and goes back to investigate. He finds a nurse wheeling a soldier through the hallway. They can't see Jack. Owen calls to report there's another spike. Gwen is searching on her own and finds a man sitting on a chair. A nurse walks by Gwen and tells the man that they're ready for him. However, as the nurse leaves, she sees Gwen for the first time. She asks Gwen why she won't leave them alone and insists she shouldn't be there, and then disappears.
On the pier, Tommy finally kisses Tosh. She thanks him and admits that he surprised her. She notes that she's older than him but Tommy figures that if he's old enough to die for his country, he's old enough to kiss her. She kisses him back and then asks what he wants to do next. They kiss again and they start to go back to her place. However, Jack calls Tosh and tells her she needs to come in and bring Tommy.
At the Hub, Jack explains that demolishing the hospital triggered the time shift. It's too late to stop the demolition and there's no way both times can exist at the same time. The box is still locked so they need readings. Jack sends Tosh and Owen to the hospital to place sensors. As they work, Owen warns Tosh to be careful. He figures she's fallen for him and Tosh admits she can be herself with Tommy. Owen says he doesn't want her to get hurt and says she'll have to say goodbye. Gwen calls Owen down to the radiology area. The files say that the Torchwood agents in that room saw a woman in armour ripping apart the Union Jack. Owen checks it out and spots an advertisement for cars showing a woman in armour, and they realize the time shift occurs now.
The lights in the hospital start going off. At the Hub, the box opens and Jack takes out the envelope inside. He reads through the papers and tells Ianto that it's instructions for Tommy… and Tosh.
Jack tells the team that in 12 hours there will be a brief moment when Tommy exists in both time zones. He needs to step back through to the past and close the rift, and Tommy will be back in hi own time for good. Inside the box is a rift manipulator that Tommy can use to seal it once and for all.
Jack meets privately with Tosh and tells her that Tommy dies three weeks after they send him back. He's shot by a firing squad for cowardice brought on by shellshock. Tosh insists they can't send him back but Jack warns that she's the only one that Tommy trusts. He shows her a drawing of herself from the box. The Torchwood agents in 1918 saw her telling Tommy what to do. Jack believes she's strong enough to do it. He admits that Tommy doesn't know what happens to him and Tosh doesn't know what to say to Tommy if he asks.
Tommy wonders what they can do while waiting until the rift appears. Tosh suggests that he come home with her and Jack says that's fine as long as it's what they both wants. At her apartment, they kiss and spend their last few hours together.
At Torchwood, Ianto and Jack discuss Tommy going back, and if Jack would ever want to go back. He admits that he's found a new home and seen things he wouldn't have otherwise, and they kiss.
Tommy wakes up Tosh to ask her what Jack told her of what happens. She lies and says he was sent back to France and they never find his body.
The next morning, Tosh wakes up Tommy and tells him that it's time.
The team arrives at St. Telio's and the rift monitor alarms go off. Tommy hears the voices from the present and the 1918 staff and patients start appearing. Tommy runs into the ward, realizes that Torchwood took him, and runs out. Tosh and Jack go after him and he admits that he knows he'll be sent back to the trenches and he can't go. He accuses them of being no worse than the generals who sent him out to fight, but Jack insists he's the only one who can do it. Tommy believes all the time he's had means nothing. Tosh asks Jack for a minute alone with Tommy. She insists that he's a hero and that they need him to save everyone. Tommy insists he wants to stay there with her. A wind picks up and they clutch at each other as the time rift opens.
Tommy and Tosh find themselves in 1918 with Gerald and Harriet. She tells Tommy to tell the agents what to do. He braces himself and then gets up and tells them to take him in 1918 so he can be there now. There's a flash of light and Tommy and Tosh are back in the present. He warns her that he'll be gone soon. They kiss one last time and he walks away back to his own time.
In 1918, a nurse finds Tommy in the supply room and sends him out.
Tosh finds the rest of the team and gets them out of the building.
In 1918, Harriet and Gerald go to get Tommy, introducing themselves as Torchwood. As they take the 1918 Tommy away, the future Tommy slips in and gets into his bed.
The team gets back to the Hub and determines that the time shift hasn't stopped. They look on as the time rifts spread out into the city. They realize that Tommy hasn't used the key yet. Jack prepares to go back, but Owen suggests that they send a psychic image of Jack through one of the rifts, using Tommy's blood sample to home in on his DNA. Tosh volunteers, saying that Tommy trusts her. Jack agrees and they inject her.
Tosh makes psychic contact with the shell-shocked Tommy, who doesn't remember what he's there to do. She manages to get through to him, reminding him that he's her hero and he's saving her future. He uses the rift manipulator and there's a flash of light. Tosh wakes up in the present and assures them that Tommy did what he had to.
Later, Tosh is packing away Tommy's clothing in his suitcase. Jack thanks her and she goes outside. Owen accompanies her and says that she was strong. He points out that everything they know still exists because of her, but Tosh insists that it's there because of Tommy, and hopes that they're worth it.