The day starts as any day would, heading out the door for work, kissing the wife and son goodbye. This day is special as it is Dan's anniversary. That evening, on his way to dinner, he dozes off, has a very strange dream (or was it) and wakes up in a different cab than the one he originally entered. Shrugging it off as a strange dream, albeit frighteningly realistic, Dan and Katie celebrate their anniversary, which later moves to a home celebration. They fall asleep in each others arms, but Dan wakes up 20 years in the past.
After stumbling through San Francisco, trying to gather some idea of what has happened, Dan is able to save the life of a distressed man, Neal Gaines, from an apparent suicide attempt. After their brief interaction, Dan quickly walks back to his house, only to find he's back in the present day, but it's been two days since he disappeared. He unsuccessfully tries to convince his loved ones what happened.
The next day, while heading to meet a source for a news story, Dan disappears again, leaving his car to cause a minor accident. Dan again finds himself in the presence of Neal Gaines, this time in a restaurant where Neal is having a discussion with his fiancee about their unborn child. The discussion gets heated and Neal leaves. Dan is placed in a position to console the fiancee, and encourage her to keep the child she is planning to abort. Almost instantly, Dan finds himself back in the present.
Dan begins to research Neal Gaines, trying to determine why his trips seem to be focused on this particular person. During his research he discovers that Neal's wife and child are killed on New Year's Eve 1997. Suddenly, Dan finds himself leaping back again. This time, Dan has an idea as to what is going on, and takes actions to help his loved ones believe him, as well as trying to figure out why Neal Gaines is so important. However, this time, when he returns to his original home, he has an opportunity to interact with his deceased fiancee, Livia. After their encounter, he leaves to find Livia standing in the hallway, which is impossible since Livia was asleep on the bed when he left. The Livia in the hallway seems to know that Dan is jumping back and forth, and refers to him as 'the one.' Dan tries to question Livia as to what is happening, but she runs away, telling him to follow his instincts.
As he chases, he comes to learn that it is Dec. 31, 1997. Dan remembers that as the date Neal Gaines' wife and son are killed, and he sets off to find them. After a unique encounter where he witnesses a lunch appointment with his brother, his brother's fiancee, Katie (who ends up marrying Dan after Livia dies), Livia and himself in the past, he learns the whereabouts of Neal Gaines. On his way to their house, he actually boards a bus with Neal, who is clearly distressed. They get off the bus near Neal's home, where Neal's wife and son are getting into a car. Neal shouts to them, and his wife, clearly frightened, orders their son back into the house. As Dan calls for Neal, Neal turns to look back at Dan when he is hit by a bus, revealing a gun in his pants. Neal's wife indicates that she was leaving because Neal had threatened to kill her and their son if they left him. Dan steps away from the accident scene and disappears.
Back in the future, Dan calls his brother to pick him up and take him home. On the way, he pulls footage of a school bus accident, where a passing doctor saves 6 students, even having to perform a field tracheotomy on one of the students. The doctor turns out to be Neal Gaines' son. Dan realizes that while he though he was leaping back each time to interact with Neal, he was actually ensuring Neal's son would survive - saving Neal so he and his fiancee would conceive, convincing the fiancee to keep the child, ultimately preventing Neal from murdering his son.
Upon finally arriving home, Dan's wife, Katie, is clearly at the end of her wits. He takes her out to the patio, reminding them that they built the patio two years after they moved in. He tears up a section of the patio to retrieve a tool box he had buried with a newspaper from Dec. 31, 1997, and Katie's wedding ring, which Dan had taken with him to be cleaned and resized for their anniversary. Suddenly, Katie realizes that Dan is telling the truth, and the he has traveled into the past.