Letting go of the past and moving on is healthy but hard. “God grant me the serenity to accept the things I cannot change,” the old prayer goes, “the courage to change the things I can and the wisdom to know the difference.”

Our very human desire for do-overs has fueled the fantasy underlying countless time-travel movies, including variants like the 2000 film “Frequency” and the 2016 CW series inspired by it. (They’re not time travel stories, per se, but they involve people in the present who can communicate with dead people in the past, sticking their fingers where they don’t belong: in the flowing waters of the time-space continuum.)

“Don’t Let Go” is pretty much exactly that scenario, replacing the magic shortwave radio of “Frequency” with a magic mobile phone. Talk about your do-overs: As with those earlier entertainments, a character in the new movie suddenly finds himself able to connect with a loved one whose death may be preventable. At least with “Frequency,” the glitch was attributed to the aurora borealis; here, it seems to be some kind of unlimited family data plan that your cellular carrier does not offer.

As unoriginal as the premise might be, the movie works well enough, thanks mainly to its two stars: David Oyelowo, portraying an LAPD detective named Jack, and Storm Reid as his teenage niece Ashley, who was murdered two weeks earlier. Their connection and chemistry — which play out over the course of the film in cellphone conversations wherein Jack tries to prevent her death — is critical to the success of the far-fetched story, as it always is with such things. Sure, it’s a sci-fi-tinged mystery thriller, but it’s also, at heart, a tale of two people. If we don’t care about them, the whole thing falls apart.

Fortunately, we do. Reid (of “A Wrinkle in Time”) and Oyelowo (of “Selma”) are immensely likable performers, and their on-screen rapport is palpable, even though they are hardly ever in the same place together, let alone the same time, save for a handful of scenes.

One early scene sets up the nature of their bond: Ashley’s father, played by Brian Tyree Henry, has bipolar disorder and is a not-quite-reformed drug dealer, so she’s looking for a more reliable father figure. Midway through the film, a shot of Jack and Ashley sharing a diner booth together — as the dead girl talks to her distraught uncle — is a metaphor, not meant to be taken literally. In another scene, a pre-murder Ashley hangs up the phone with future-Jack, only to chew out past-Jack, who has a look of confusion on his face that may mirror the one on yours.

“Don’t Let Go” manages, at times, to generate a nicely weird “Twilight Zone” vibe, but fails to sustain it, as it also runs into some of the same problems that plague movies of this ilk: If you tear the fabric of time by altering what has already happened, it can be difficult to sew it back up straight. This makes for a narrative that is occasionally muddy and ambiguous, as Jack issues instructions to Ashley with the benefit of hindsight that isn’t quite 20/20. He know what happened, in other words, but he can’t predict what will happen when he undoes it.

This leads to circumstances that put him — and Ashley — in sometimes violent jeopardy, as he tries to simultaneously save the girl, by selectively plucking out threads of the past, and unravel the whodunit. His efforts make for a watchable movie, if one that’s less than deeply satisfying, not to mention observant of the laws of logic.

Emotionally, “Don’t Let Go” works like magic. Intellectually, not so much.