Frequency

In 1969, veteran firefighter Frank Sullivan dies on duty and is survived by Julia, his wife, and John, his young son. In 1999, John has become an NYPD detective and struggles with emotional openness as a result of his loss. An atmospheric phenomenon causes a broken radio in John's home to start working again, and he realizes he can communicate with Frank in 1969 through it. John forewarns his father of his imminent death, allowing Frank to avoid the accident that would have killed him in 1969.
John experiences new memories as a result of changing the past, now remembering Frank dying in 1989 from lung cancer. He shares this news with Frank in 1969, who promptly quits smoking. Changing history brings unintended side-effects: Frank's survival allowed Julia to unknowingly save the life of a serial murderer known as "the Nightingale", who now kills 10 victims, including Julia, as compared to three in the original timeline.
John shares the Nightingale's case history with Frank, allowing him to save some of the would-be victims in 1969, but the Nightingale steals Frank's driver's license and plants it on his next victim. By sequestering potential fingerprint evidence in their house, Frank passes physical information from 1969 to 1999 to help John stop the Nightingale, who is revealed to be NYPD detective Jack Shepard. Frank is arrested by his close friend Det. Satch DeLeon (John's boss in 1999) on suspicion of murder, and attempts to prove himself innocent by predicting play-by-play details of the upcoming 1969 World Series Game 5, which he heard John recount.
Frank escapes from the police station and breaks into Shepard's apartment, where he finds trophies from the murders. Shepard arrives and attacks Frank, but is seemingly killed after a scuffle and a fall into a river, allowing Julia to survive until 1999. Frank's predictions of future events prove to be accurate, letting Satch realize he was telling the truth. Satch investigates Shepard's apartment and exonerates Frank, but the police find no trace of Shepard in the river.
Frank returns home, where he and John are attacked by separate versions of Shepard in their respective times. In 1969, Frank blows off Shepard's hand with a shotgun, causing the same hand to dematerialize in 1999 as Shepard fights John. Shepard in 1969 escapes, history changes again, and a now-living Frank in 1999 comes to John's rescue, killing Shepard and resolving both time frames.
The film concludes with John happily married and with children, enjoying the company of both his aging parents, and witnessing his acquaintances benefit from future information he shared with their past selves.