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A feat of imagination surrounding the very real agreement that closed one of the widest chasms in political history, “The Journey” proposes that the bloody, hard-fought Troubles in Northern Ireland may have been settled with that most timeworn of movie tropes: opposites stuck together on a road trip.

This fictionalized account of firebrand Protestant loyalist Ian Paisley (Timothy Spall) and ex-IRA leader/Sinn Fein politician Martin McGuinness (Colm Meaney) going from zeros to heroes while being driven to an airport is the kind of rift-bridging uplift that makes cynics wonder who else might have been served by such a scenario: Aaron Burr and Alexander Hamilton on a cross-state carriage trip? Donald Trump and Rosie O’Donnell sharing a golf cart?

Those are glib jokes, sure, but filmmaker Nick Hamm, a former resident director with the Royal Shakespeare Company, isn’t above using natural warmth and humor to find just how never-concede enemies reached the point where, according to the end text, they came to be known as “The Chuckle Brothers” for the fondness they clearly shared in public once they began leading Northern Ireland together.

If anything, it’s an over-reliance on contrived points of debate and a-ha moments of camaraderie and gentle comedy that keep the film from being a truly enlightening, deep-dive example of the kind of big-conversation drama that lays bare the inner workings of major historical figures.

The basis for the movie’s premise is a truth about the 2006 peace talks in Scotland: Paisley, a Free Presbyterian minister, needed to get back to Belfast for his 50th wedding anniversary, and McGuinness — perhaps inspired by a tradition of Northern Ireland politicians sharing the same vehicle to avoid assassination attempts — insisted that he be on the jet, too. For cinematic reasons, Hamm and Bateman shifted the action from a plane to the ground.

The prospect of these figures in same-oxygen proximity animates U.K. prime minister Tony Blair (Toby Stephens) and an intelligence official (John Hurt) enough that the van is rigged with surveillance equipment so representatives from both sides can monitor the conversational progress. Even the young chauffeur (Freddie Highmore) is a security plant, meant to cheerily get the two talking and take detours to extend the men’s time together.

At first, McGuinness, like a man with a pint in his hand and a joke to tell, tries to crack Paisley’s icy veneer with mildly barbed small talk, while the reverend shuts his eyes as if nobody else is in the cab with him. This is Meaney and an artificially aged Spall at their best, turning raised eyebrows and grunts into language.

But as the topics of intransigence, conscience and blame for bloody conflict stoke the pair’s fire, it feels less like a real philosophical back-and-forth and more like didactic turn-taking. “The Journey” is certainly palpable fantasy, especially in Meaney’s and Spall’s hands. Absent being present when peace is negotiated, we need thoughtful art to fill in the sights, sounds and textures of violent hatred defused — the words that reinforce but then turn the corner, the gestures that seem suspicious before they make sense.

But while the filmmakers have the right idea overall, their love of contrivance too often gives “The Journey” the sense of being reverse-engineered to explain a breakthrough rather than driven by the messy, human possibilities of their what-if.

The Journey

★★ out of 4 stars

Rating: PG-13, for thematic elements including violent images and profanity.

Theater: Lagoon