Despite a prestigious literary pedigree—it’s based on a well-received novel by John Burnham Schwartz and co-produced by the new theatrical arm of his publisher, Random House Books—and a formidable adapter (Terry George, of “Hotel Rwanda”) and cast, “Reservation Road” feels like a movie that’s escaped from the Lifetime Network—and one that might even benefit from commercial interruptions. It deals with powerful personal issues—the grief that follows the loss of a child, the guilt that tortures a basically good person who knows he’s done something terribly wrong—but treats them through a clumsy plot twist, based on the sort of coincidences that one can swallow on the printed page but can’t help but feel phony on screen, that cheapens them. The result is a sad disappointment.

Joaquin Phoenix stars as Ethan Learner, a New England academic with a lovely wife, Grace (Jennifer Connelly), and two doting children, ten-year old Josh (Sean Curley), a musical prodigy, and Emma (Elle Fanning). When the family stops as a gas station on their way home from a park concert in which Josh has shown his skill on cello, the boy wanders near the highway to release a jar of fireflies and is struck and killed by an SUV driven by Dwight Arno (Mark Ruffalo), a divorced dad who’s bringing his son Lucas (Eddie Alderson) back from a Red Sox game. (The tale is set during the team’s pennant drive in 2004.) Dwight, who’s already late getting the boy to his ex-wife Ruth’s (Mira Sorvino) place, doesn’t stop, and the case becomes a hit-and-run.

The grief-stricken Ethan becomes obsessed with finding the driver and, when the police fail to do so, takes matters into his own hands, visiting Internet chat rooms to share his feelings with other parents who’ve lost children in road accidents and even contacting a law firm to push the investigation. Dwight, meanwhile, is consumed by guilt even as he tries to avoid arrest and keep up the connection with his son through the Sox’s run.

So far, so good. The problem with the script is that the characters are brought together by contrivances that, in this context, strain credulity. Dwight happens to be an attorney at the firm that Ethan hires, and he’s assigned the case—inevitably coming across as nervous in Learner’s presence and eventually exciting his suspicion. And that’s not all: Ruth is Josh and Emma’s music teacher, and school events will inevitably bring the families together.

In a novel, where the story is spread over hundreds of pages, these sorts of coincidences can be made plausible, or at least tolerated; on the screen, they come across as extremely unlikely. Perhaps the film could still have worked had the performances been extraordinary, but though Phoenix and Ruffalo are both fine actors, they’re far from their best form here. The former suffers nobly, and he has a couple of good scenes with the underused Connelly, but he never achieves the sort of searing intensity the part requires—especially not in the inevitable final confrontation with Ruffalo, which unfortunately takes a melodramatic turn. And Ruffalo, for his part, never rises above a curiously one-note pathos that seems simultaneously overplayed and undernourished; his best scenes are with young Alderson, but even there he never quite connects. Since “Reservation Road” is essentially a two-man show—the rest of the cast, even Connelly and Sorvino, are clearly doing supporting work—their weakness is fatal, especially since George, as in that final face-off, doesn’t manage to give the film the kind of power he did “Rwanda.”

Nor does he make great use of the Connecticut locations, which he and cinematographer John Lindley fail to endow with much personality. Nor does Mark Isham’s score capture the sense of muted sadness the tale should generate.

“Reservation Road” is undoubtedly a sincere film, but it never succeeds in achieving the visceral force such a story should convey. For a picture about a child’s hit-and-run death, it’s an oddly bloodless affair.