As a young child, Rose is called in to her mother's bedroom. Jackie shows her photos of her dead husband, Pete, who passed away on September 7, 1987, when Rose was just a baby. She explains that Peter was always having adventures, and Rose would have loved to seen her then.
In the present, Rose relates to the Doctor what Jackie told her, and suggests that he use the TARDIS to take her to see her father. He wonders why and assures her that he can do anything, but warns her to be careful what she wishes for. They go back in time and watch as Pete and Jackie are married, and Pete stumbles over her name. Rose admits she thought her father would be taller, and remembers how Pete died alone after he was hit by a hit-and-run driver.
Back in the TARDIS, Rose asks the Doctor to take her to the day of Pete's death so that he doesn't die alone. He activates the controls and takes the TARDIS to 1987, and asks one more time if she's sure. They look at the road where Pete died and Rose describes how Jackie told her that Pete got out of his car after getting a vase for a wedding and was hit. They watch as Pete arrives, and the Doctor takes Rose's hand as a hit-and-run driver strikes Peter, killing him. The Doctor tells Rose to go to him, but she can't go through with it until it's too late. She asks the Doctor for another chance. They go back and watch as their earlier versions watch Pete get hit. Rose shoves past them, grabs Pete, and pushes him to safety. Their earlier versions disappear, and Rose is relieved and surprised that she saved her father. She introduces herself and he says he's going to Sarah Clarke's wedding and offers her and the Doctor a lift. The Doctor watches in horror at what has happened.
High above London, something flies over the city, seeking prey.
Pete brings Rose and the Doctor home while he gets ready. Once they're alone, Rose looks around at all of Pete's inventions. The Doctor says nothing at first, and then wonders if she came along with him only because she knew he could travel in time, and dismisses her as another stupid ape. He explains that two sets of them being there made it a vulnerable time, and that there are some things that can never be changed. Rose wonders if the Doctor wants Pete dead, and figures he's jealous. He demands her TARDIS key and starts to leave, but Rose says that she knows how lonely he is and he won't make good on his threat.
The Doctor goes to the TARDIS, unaware that something overhead is watching him. As he walks along, the creature above flies down and starts killing people.
Rose tries to clean up Pete's apartment but he asks her to stop and asks about her boyfriend. Rose insists that they're not a couple, but worries that the Doctor left her. She refuses to discuss it with Pete and then offers to go with him, and he figures that she's nut. However, he can't help thinking he's seen her somewhere before.
At the church, Stuart looks nervously around the church and notes to his father Sonny that a lot of people haven't shown up. Sonny suggests that his son think about it, and then they both notice that the temperature has suddenly dropped.
The Doctor arrives at the TARDIS and opens the doors, only to discover that there is nothing inside but an empty shell. Realizing that Rose is in danger, he runs to find her.
As they go to the church, Pete talks about his business ventures. When Rose is less than impressed, Pete wonders if she's been talking to his wife Jackie. Rose claims that Jackie described him as the most wonderful husband in the world. As they drive, the radio changes to a more recent music. Rose tries to call on her cell phone, but gets a voice saying, "Come here, Watson, I need you" repeatedly. Behind them, the hit-and-run driver approaches their car, drives by them, and then disappears.
At the church, Sonny is making a call when the same phrase repeats on his phone. Stuart comes out and tells him to come inside before he sees the bride. Sarah arrives, but her bridesmaids Suzie and Bev tell her to go around the block because so many people are arriving. Jackie asks Bev to take her baby daughter Rose and keep an eye on her. Pete and Rose arrive and are almost hit by the hit-and-run driver when it mysteriously appears ahead of them. Pete narrowly avoids it as Rose calls out "Dad," and he wonders what she's talking about. Rose can't help staring at Jackie, and is surprised to see it in a new style. She's shocked to see herself as a baby. Meanwhile, Jackie assumes that Rose is one of Pete's mistresses and soon starts arguing with her husband, saying he's a failure. Rose finally tells them to stop, saying that they are supposed to love each other. They both walk away.
At a playground, the creatures attack again. One young boy, Mickey Smith, notices them disappearing one by one. His mother disappears from behind him without a trace, and he runs off in. fear.
Rose watches as Pete tries to reassure Jackie. He insists that one day he'll get things right. Mickey arrives and runs into the church, talking about monsters. The Doctor comes running up and tells everyone to get into the church. A black winged creature appears in the sky above Rose and attacks her, but the Doctor pulls her away and gets everyone into the church. More of the creatures appear above them. Sonny panics and runs off, and one of the creatures consumes him. Another one attacks the reverend, while a third flies away from Sarah when she screams. The Doctor gets them all into the church and closes the doors.
Inside, the Doctor explains that the older something is, the more resistant it is to the creatures, the Reapers. He tells Jackie that the creatures were created by an accident in time, and they are like bacteria taking advantage of the wound. Jackie wonders how he knows her, but he orders her to do what he says. Stuart shows the Doctor the phone and the voice, and the Doctor explains that it's Alexander Bell. He then tells everyone that the creatures are there to sterilize the wound. When Rose wonders if she's responsible, the Doctor merely turns away.
Outside, the creatures continue to prey on everyone, consuming them. As they barricade the church, Pete figures that they are spreading across the world and killing everyone. The Doctor notices the hit-and-run driver go by again and then disappear, and tells Pete not to worry about it.
As the creatures probe at the church for a weakness, Pete approaches Rose and admits that he would never give the keys to a complete stranger. He wonders why he trust her and why she called him Dad. Pete realizes that she's his daughter and embraces her.
Stuart and Sarah turn to the Doctor for help. He realizes that Sarah is pregnant and asks them how they met. He insists that they're important and promises to save them.
Pete wonders how Rose got there, and she explains that she came in a time machine. He asks what the future is like and what happened to him, and he realizes that he's dead. Switching subjects, he asks if she has a boyfriend. Young Mickey comes running in and hugs Rose, and Rose realizes who he is. Jackie comes in after Mickey and figures that Rose is with Pete because she's his mistress. As she storms out, Rose warns Pete that he can't tell Jackie what he knows.
In the chapel, Jackie has the Doctor watch the baby Rose. He warns Rose not to touch her younger self, as the resulting paradox would make the Reapers stronger and let them in. He finally apologies for snapping at her and admits he wasn't going to leave her. The Doctor then admits that he doesn't know of any way out, and the reapers will eventually get in and kill them. The Time Lords would have stopped such threats, but they no longer exist. He asks her to apologize, and Rose does. The Doctor smiles and embraces her, and they realize the TARDIS key is glowing. He picks it up and explains that it's still connected to the TARDIS. The Doctor explains to the survivors that he can use the key to access the TARDIS, and uses the battery in Sonny's phone to charge it up.
As the creatures circle outside, Pete asks Rose why she came to that time period and she claims it was an accident and her saving him was a coincidence. He returns to the subject of his future and Rose lies. Pete wonders if he's a good dad and she describes an ideal father. He realizes that it's not him. Before he can pursue the matter, they watch as the TARDIS begins to reassemble itself around the TARDIS key. The Doctor tells everyone not to touch the key, and they'll manage to escape.
Outside, the hit-and-run driver goes by again and disappears in a flash of light.
As Jackie holds baby Rose, Rose and the Doctor watch her. He explains once things are back to normal, they won't remember anything but Pete will be alive. Pete realizes what they mean and insists that he's useless. Jackie overhears them and misunderstands, thinking Rose is his daughter with another woman. He insists it's the same Rose and hands the baby to Rose before the Doctor can stop him. A Reaper appears in the middle of the church and the Doctor orders everyone behind him. He tells the creature to atack him as the oldest thing there, and it consumes him. It flies through the materializing TARDIS and it and the TARDIS disappear.
Rose picks up the TARDIS key and discovers that it's cold. She blames herself for destroying the whole world. The other Reapers gather at the windows as they realize they're doomed. Pete looks outside and sees the driver appear and disappear again. He then tells Rose what he's seen and that the Doctor knew what it meant, but tried to protect him to spare Rose's feelings. Pete then tells Jackie to look at Rose and realize who she really is. Mother embraces daughter and Pete explains that he's meant to be dead. He says she has to survive so she can bring up their daughter, and they kiss. Pete tells Rose that he wouldn't have been there for her if he'd lived, but he can die for her and be a proper dad. He picks up the vase and thanks her for saving him, and then embraces his family one last time.
Pete runs outside and sees the car appear. He runs in front of it and is hit, and the Reapers disappear. At the church, the Doctor returns and tells Rose to go to her father. She runs to where he lying in the street. Pete looks up at her and smiles, and then dies. Everyone has been restored with no memory of what happened. Now Rose remembers Jackie telling her that Pete just ran in front of the car, and that an unknown girl sat with Pete and held his hand as he lay dying, and then disappeared.
The Doctor takes Rose back to the TARDIS, as she remembers Pete, the most incredible dad in the world, dying on September 7, 1987.