For as long as there have been romantic comedies, there have been erstwhile lovers making convoluted bets and flirty deals with so little basis in reality that they might as well be science fiction. When was the last time you heard a real person use the phrase “If neither of us are married by 40, we’ll marry each other,” despite hearing characters say as much over and over again in movies and TV shows? It’s the kind of gamble that fictional stories love to make by turning feelings that are far more likely in the real world to remain sexy subtext into blunt text, flat-out daring its characters not to fall in love with each other. (Completely unsurprising spoiler alert: They always fall in love with each other.)

“Run,” HBO’s new romantic(ish) comedy from “Fleabag” producer Vicky Jones, takes this convention to a whole new extreme. After dating through college and then drifting apart to live their lives, Ruby (Merritt Wever) and Billy (Domhnall Gleeson) made an unusual pact: If either of them ever sent a message reading “RUN,” and the other responded with the same within 24 hours, they’d drop everything else in their lives to meet at New York City’s Grand Central Station, hop on the first cross-country train leaving after 5 p.m. and spend one no-holds-barred week together. Despite some close calls with one being more tempted than the other, their desires don’t quite sync up until 17 years later, when Billy texts Ruby and kick-starts a chain of cataclysmic events for which neither is at all prepared. Equally terrified and exhilarated by their mutual choice to run away together, Billy and Ruby try their damnedest to shed their disappointing lives to be better versions of themselves — or whatever “better” means to them, anyway.

Ever since Phoebe Waller-Bridge’s “Fleabag” swept the Emmys, it’s become something of a trend to compare it to absolutely any project with a sardonic woman — which Ruby, despite the baby voice she uses to placate her husband (Rich Sommer), certainly is — at its center. “Run,” however, can lay a stronger claim to that parallel than most; Jones has long worked on “Fleabag” and “Killing Eve” alongside Waller-Bridge, who in turn acts as an executive producer (and eventual guest star) here. But even though Ruby is smart (and a little mean), and Billy is a charming Irishman (with a low-key drinking problem), Ruby isn’t Fleabag and Billy isn’t the Priest. They are their own characters, bruised and spiky and proud in their own particular ways.

“Run” does, however, suffer from leaning too hard on its leads’ (admittedly scorching) chemistry rather than tightening its story, which quickly takes some needlessly complicated turns. For one, the show doesn’t fully explain the terms of Ruby and Billy’s extremely specific agreement until a few episodes in, which makes for a confusing introduction. For another, Ruby and Billy’s extremely specific dynamic is so immediately compelling that when “Run” throws in a new and much-higher-stakes element deep into the season, it’s almost disappointing to realize how hard of a turn it’s taking from the already compelling blueprint of their relationship drama.

Forged in cutting banter and knowing grins, Ruby and Billy’s singular connection — and constantly simmering sexual tension — is obvious to anyone who sees them. It doesn’t hurt that Wever and Gleeson make a five-course meal out of every stolen glance, buoyed by Gleeson’s lethally charming portrayal of Billy’s neuroses and Wever tearing into Ruby’s latent fury with palpable relish. Not many actors could anchor a show that’s practically a two-hander (with the exceptions of Sommer and a slyly unhinged Archie Panjabi as Billy’s business partner). But Wever and Gleeson are up to the challenge, and it’s undeniably, wildly fun to watch them embrace it.

As per Jones’ pithy pilot and Wever’s portrayal, Ruby is a refreshingly new kind of disaffected housewife. She seizes the opportunity of Billy’s message (after a flirtation with having a panic attack) to shed her reluctantly honed docile PTA mom exterior and unleash the sarcasm and confidence lurking just beneath. Gleeson’s Billy, meanwhile, is wry and wounded and officially disillusioned with his career as a motivational speaker — which makes Ruby’s uncanny ability to call him out when he’s being disingenuous all the more alluring.

Ruby and Billy would be drawn to each other no matter what, but given that the first four episodes largely take place on an Amtrak, they’re quite literally forced into each other’s space in a way that makes even the most mundane situations funnier and more animated. (The scene in which they try to act on their lust, for example, is a hilarious screwball nightmare.) Director Kate Dennis (“The Handmaid’s Tale”) finds a way to make the camera practically jog to keep up as Ruby and Billy zip down aisles and around each other’s rising charm. When Dennis highlights the unspoken link between them by following Ruby and Billy’s charged eye contact in suggestive close-ups, or in lingering shots that track one watching the other undetected, even their most ridiculous moments have the electric frisson of an erotic thriller.

Just by tracing the ins and outs of Ruby and Billy’s attraction to each other, not to mention their roiling discontent with themselves, “Run” more than lives up to its name with the kinetic, slightly frantic energy of a train tearing down (and nearly off) the tracks. It doesn’t need a drastic eleventh hour twist to keep things interesting when it already has that.

“Run” premieres April 12 at 9:30 pm on HBO. (7 episodes; 5 reviewed.)