In the hyper-connected modern world of everyone extremely online, the one that got away is an obsolete concept. He/she/they is perhaps only just one text message away and perhaps just one step away from daring you to jump off the cliff of normalcy into blissful self-destruction. So the longing romantic fantasy of what if?, underscored by the underpinnings of life’s routines and inherent romantic dissatisfactions, is explored with scintillating, flirty, flinty pleasure in HBO’s romantic comedy cum thriller mini-series, “Run.” From creator Vicky Jones, a frequent collaborator of “Fleabag” creator and star Phoebe Waller-Bridge, also an executive producer and guest star in the series, and starring Merritt Wever (“Unbelievable,” “Godless”) and Domhnall Gleeson (“Ex Machina,” “Rise Of Skywalker”), ”Run” has all the trappings of a terrifically entertaining series about secret romantic pacts, unfulfilled desire, star crossed destinies, and perhaps the manic thrill of blowing up your life all in the name of a sensual romp between two people with electric chemistry.

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Alas, “Run,” which comes on extremely strong— deliriously frisky and intoxicatingly coquettish—loses its breath quickly, turning from delectably horny wish-fulfillment dramedy into a high concept thriller with dark twists, as the series explores a romantic situation gone horribly awry with dangerous, life-altering consequences.

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What keeps “Run” largely on course is the absolutely outstanding performance by Wever, coming off her small, but magnetic performance in “Marriage Story,” as well as series such as “Unbelievable” and “Godless.” Wever is clearly having a moment, and she’s electrifyingly engaging here, demanding your attention at all moments. (Note to someone in the Baumbach/Gerwig household, please write a lead role for her asap).

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The limited series follows Ruby Richardson (Wever), whose unexciting, sexless life is upended one day, when, out of the blue, she receives the text message “RUN.” It’s from her college sweetheart turned self-help guru Billy Johnson (Gleeson, always great, but genuinely outclassed here) teasing the fulfillment of a treaty they made 17 years ago. The pact said, should either of them receive a text with the message “RUN,” they would drop everything in their lives, meet in Grand Central Station, and travel across America together.

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Both at a crossroads that reveals itself over the course of seven episodes, Ruby—married with kids—doesn’t hesitate, dashing out the door filled with the terrifying thrill of escaping one’s life, having a little ex-sex without really weighing all of the consequences. Billy is in a different circumstance, at a rock bottom of sorts after a self-help seminar goes awry and viral, potentially incinerating his career.

Each person in the tryst is especially coy with their life stories, having apparently not really Googled where the other is in life, but that’s part of the enamoring allure of their little kittenish game. And again, Wever is just lip-bitingly entrancing and enticing—you too want to just immediately jump on a train with her, play goo goo eyes and hit the sheets immediately. Their chemistry is electric, and Wever’s unspoken unfulfillment in life, seemingly bursting with yearning and desire simply crackles with scrumptious energy.

Their flirtatious cat and mouse, will they or won’t they seal the deal game—and how long it can be sustained before Ruby deals with the alarming freakout of her husband—however, is upended by the secrets in Billy’s backstory. They’re not all that mysterious, really, but they’re played as such, so let’s just say they involve Fiona (Archie Panjabi), Billy’s former personal assistant.

Soon, “Run” takes an ominous turn and near the end of the series, an extremely shocking one, that just derails the amorous, frothy lightness of the show and turns it into a much darker, unexpected thriller. By this point in the series, credulity has already strained and the initial ecstatic kiss excitement of the concept has begun to fade. (Waller-Bridge cameos briefly here too, but her entire appearance feels unnecessary and forced.)

What “Run” excels is its electricity; the mix of breathy, nervous anticipation energy between the two ex-lovers—especially in the first two delightful episodes—and the correlating notions of dissatisfaction, disillusion, unhappiness, how the two are directly related and fuel one another. And again, the very profoundly real emotional impact of it all that Wever expresses so well, including the ideas of both the anxieties of motherhood and how motherhood doesn’t mean the end of desire or the carnal lusts of being wanted. There’s a train genre element too—everyone is trapped inside, within their own world, escaped from reality and disconnected from the outside which creates that illusory bubble that everyone can be naughty during this little escapade.

But even at a brief seven episodes, all that heart-palpitating rushes of manic yearning, fear, lust, and expectation can only sustain themselves so long. The dark crush of reality eventually sets in, the genre twists start to take hold and like bubblegum, the flavor quickly dissipates. Fortunately, “Run” isn’t a marathon and while it can’t maintain the magnetically-charged voltage of its initial sprint, it can be quite enchanting. Meanwhile, the seductive charms of Merritt Wever are on full display, a performative treat if there ever was one, and a further confirmation of what a real deal actor she is, poised for so many great races ahead. [B-]