Plot
In 1969 New York, a gasoline tanker overturns on a highway ramp, spilling fuel into an electrical substation below ground and trapping two workers. Among the responding firefighters is veteran Frank Sullivan, who goes underground to rescue the workers against the direction of his commander. Despite the rising level of fuel and the sparking created by damaged electronics, Frank and another firefighter pull the workers out and escape just before a spark ignites a huge explosion. Frank returns safely home to his wife Julia and young son Johnny.
In 1999, John Sullivan is an NYPD detective still living in his childhood home. His girlfriend Samantha leaves him because she believes he is emotionally shut off, the result of his father Frank dying in a fire when John was six years old.
John's lifelong friend and neighbor, Gordo, stops by to borrow fishing equipment and finds a Heathkit single-sideband ham radio that once belonged to Frank, but fails to get it working.
The night before the 30th anniversary of his father's death, John is surprised to find the radio operating during a particularly intense occurrence of the aurora borealis, and has a brief conversation with another man concerning the 1969 World Series, which John is able to recount in specific detail.
He realizes from this and other details that he is communicating with his father on the same date in 1969, and warns him of the mistake that led to his death. At first, Frank is alarmed and refuses to believe, but the next day, while rescuing a runaway from a burning warehouse, he recalls John's warning, changes his course, and survives the fire.
In 1999, John is suddenly struck with new memories of his father surviving until 1989. That night, he and Frank reconnect, and they share details of their lives. John warns Frank that he will eventually die in 1989 of lung cancer. With no further comment, Frank throws his cigarettes in the trash.
The next day, John discovers that his present has been changed in other unexpected ways. He and Frank have inadvertently prevented the death of the "Nightingale", a serial killer who originally murdered three nurses in the 1960s and was never caught.
In the new modified reality, the Nightingale has now killed ten women, including John's own mother Julia. John is now flooded with new memories of his mother's death. To save her and the other future victims, John enlists Frank's help in stopping the Nightingale before he can kill again. At first Frank begs off, claiming "I'm just a fireman," but John is insistent.
Following information from John about the killings, Frank saves the first victim, but before he can rescue the second, the Nightingale subdues him, steals his driver's license, and plants it on the next victim to frame Frank for the murder.
When Frank relates his experience, John realizes Frank's wallet has the Nightingale's fingerprints. John asks his father to hide the wallet somewhere in the house where John can find it 30 years later. Using the preserved fingerprints from the wallet, John identifies the Nightingale as Jack Shepard, a former NYPD detective.
In the original timeline, Shepard was a hospital patient under the care of Julia; when she left during her shift after learning of Frank's death, an inexperienced doctor accidentally administered a medication that caused a fatal allergic reaction. Because Frank survived, Julia was there to prevent the error, thereby saving the Nightingale's life -- but allowing her to become a subsequent victim.
In 1969, Frank's close friend, and John's future boss, homicide detective Satch DeLeon comes to Frank's house to arrest him on suspicion of murder; he resists, and the radio is knocked over and damaged.
At the station, Frank attempts to explain to a highly skeptical Satch that he got his information about the Nightingale from speaking to his grown-up son John over the radio. He proves his innocence to Satch by accurately predicting the course of the 1969 World Series Game 5, including the highly unusual shoe polish play.
Escaping from the police station, Frank breaks into Shepard's apartment, where he finds trophies from the murders. Shepard arrives and attacks Frank, who fends him off and, apparently, kills him after a foot chase and plunge into the river, thus ensuring Julia's survival. Satch, having realized that Frank was telling the truth, goes to Shepard's apartment and finds the victims' jewelry, exonerating Frank. At the same time, the police search the river for Shepard's body, but find nothing.
Frank returns home and repairs the radio. While talking over the radio that night, Frank and John are each attacked by the 1969 and 1999 versions of Shepard. In 1969, Frank blows off Shepard's right hand with a shotgun and he flees. In 1999, as Shepard pulls a gun on John and prepares to fire, the house changes rapidly around them and Shepard's hand disappears. At that point, a now mid-60s Frank appears in the doorway and kills Shepard with his shotgun. He and John tearfully embrace, the timelines now repaired.
The film concludes with a softball game including John and his own little son, his now-wife Samantha (expecting another baby), his healthy and happy gray-haired parents, Satch, and Gordo, who has now become wealthy thanks to a stock tip fed to him in the past by John -- posing as a magical "Santa Claus" -- advising six-year-old Gordo to remember the "magic word, Yahoo," which convinces him to invest in the internet company in his adulthood.
