The late composer Stephen Sondheim’s biggest career flop has turned out to be one of the best musicals of the theater season — a mere 41 years after it first premiered.

Downtown at New York Theatre Workshop, the lovable and intimate revival of his “Merrily We Roll Along” that opened Monday night lifts its audience’s spirits even during a show that, as written, is a sour cautionary tale about how life sucks more as you age.

It’s the opposite of most of the schlock that’s squatting on Broadway these days. This musical is intelligent, hilarious, human, heartfelt and alive. It makes you — gasp — think. And it has the electricity of a major event, as we are gripped and gutted by its main characters’ existences short-circuiting before us.   

Merrily? Yeah, right. The sad show depicts a friendship disintegrating backwards — portrayed by Jonathan Groff, Daniel Radcliffe and Lindsay Mendez — “Memento” style. Usually, you walk out at the end depressed and introspective and whispering about the piece’s structural flaws.

Fans have grown to cherish the score with gorgeous songs such as “Not a Day Goes By” “Good Thing Going” and “Old Friends,” but as Sondheim’s music awkwardly pairs with George Furth’s retro and oddly phrased book, productions usually go off the rails. 

But I felt differently at the end of this “Merrily,” with no-frills direction by Maria Friedman and a midcentury modern set by Soutra Gilmour.

When Groff, who is sensational playing a famous composer named Franklin Shepard, stares out at the audience in this staging’s final image, he’s back again in middle age with his life in tatters. At that moment, you really believe that he’s changed and that these three formerly inseparable New York pals can make it work; that our experience watching the show has been Frank’s own all-important lightbulb moment. An infamously unfulfilling show suddenly satisfies. 

But it’s not only a striking image that helps “Merrily” roll along, but a stunner of a trio. The old friends are fabulously played by Groff, Radcliffe and Mendez, who never overplay a millisecond of a story that can easily get caught up in showbiz cliches and 1970s kitsch. Here, everything is grounded and real.

“Merrily” starts at a party in Franklin’s Los Angeles home in the 1970s, attended by his wife, mistress and his old pal Mary (Mendez) alongside the usual Hollywood clingers. Mary is a theater critic and a raging alcoholic (thanks, guys) who causes a scene by loudly guilting Frank about his estrangement from their pal and his lyricist Charley (Radcliffe). 

And then we rewind to the 1960s. Charley blows up at Frank during a national TV interview, and Radcliffe sings the man-about-to-snap patter song “Franklin Shepard Inc.,” accusing his creative partner of selling out for money. The British actor gives one of his warmest and most honest performances yet, and delves into deeper places than “How To Succeed in Business Without Really Trying” and “The Lieutenant of Inishmore” allowed him to. The guy breaks your heart. By the way, Radcliffe has turned into such a reliably terrific stage actor, we don’t need to bring up his wand-wielding past anymore.

Throughout all the turmoil of broken marriages and spats, Mary, who dreams of becoming a novelist, is the workhorse who tries to keep their family unit together. Mendez is exquisite in the part from the offset. A less skilled actress warps older, alcoholic Mary into a crone a la the Witch from “Into the Woods.” Mendez is totally genuine and, as a result, devastating.

The same is true of Katie Rose Clarke as Beth, Frank’s first wife, who wails “Not a Day Goes By” as her husband’s skyrocketing fame leads him to stray from their marriage. As his new wife, Gussie, Krystal Joy Brown is sinuous and a master manipulator. And like he was in “Tootsie,” Reg Rogers, as the Broadway producer Joe, is a perfect encapsulation of casting-couch smarm.

The second act of the show is always less compelling than the first because the triad’s youthful career milestones — their first Broadway hit, an early downtown revue, working awful day jobs — have less oomph than the relationship dramas. What makes those staccato events succeed here is that these three actors, all being in their thirties, effectively straddle their characters’ states of being at the beginning and end. When they get their first gig, there is a palpable lived-in euphoria they emanate that will make every ticket-buyer remember their own.

Has “Merrily” finally been fixed? Not really, and it never will be. But its storied problems have not only become part of its lore, but also a messy asset in our dismal era of mechanical, assembly-line musicals produced by movie studios. “Merrily” is an affecting oddball that hits you hard emotionally, even when it creatively misses. Here’s to it. What’s like it? Damn few.