Nostalgia for the ’90s has a special double potency. The decade is now old enough that we can look back at it with the same bittersweet remember when? tug that powered ’80s nostalgia and ’70s nostalgia — and, before that, the 1962 nostalgia of “American Graffiti” (the movie that was the true launchpad of nostalgia culture). There’s a great deal in the ’90 to get nostalgic about: “Seinfeld” and Tarantino, “Sex and the City” and “Boogie Nights”; the Clinton boom and the Clinton scandals; the return of martinis; the rise of boy bands and grunge, Snoop Dogg and Eminem; the earthquake that was O.J.; and on and on. Yet part of the decade’s uniqueness came from what wasn’t there. It was the last moment before the Internet took over, and along with it a certain aura of technological control. I’d argue that with rose-colored hindsight (and when it comes to nostalgia, is there any other kind?), that’s part of what made the ’90s so bold and alluring, so relaxed and unhinged. So free.

Trainspotting,” Danny Boyle’s movie about four Scottish wastrels throwing their lives away on heroin, came out in 1996, and if you go back and watch it now, every squalid frame of it pulses with the freedom of the ’90s. The movie is set in the gutter, in icky sordid flats and junk dens, in “the worst toilet in Scotland,” yet the entire thing is recklessly and thrillingly alive. It’s a tale of addiction that’s really a movie about the liberation of not giving a fuck.

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The famous opening sequence, with Ewan McGregor — pale and buzzcut, as if he’d just escaped from a psych ward — and Ewen Bremner dashing through the streets of downtown Edinburgh pursued by the cops, with “Lust for Life” pounding on the soundtrack like a punk heartbeat and McGregor’s Renton listing all the things you’re supposed to “choose” that he would rather die than choose (“Choose life. Choose a job. Choose a career. Choose a family…Choose fixed-interest mortgage payments. Choose a starter home. Choose your friends…Choose sitting on that couch watching mind-numbing spirit-crushing game shows stuffing fucking junk food into your mouth”), is a catharsis, because it’s an embrace of life in the guise of an unholy rejection of it. It’s like the opening of “A Hard Day’s Night” rewritten by Charles Bukowski (or, in this case, by his inheritor Irvine Welsh). The characters in “Trainspotting” reject everything except getting higher than high, yet in scene after scene we taste their hunger, the nakedness of their desire for something outside the realm of what life has promised them. They’re raffish lowlifes who have slipped away from having a dream. But they’re also everyone living the middle-class dream who knows it’s a mirage.

The Danny Boyle who decided, 20 years later, to make a sequel to “Trainspotting” is a different director now than he was then. “Trainspotting” was only Boyle’s second feature, and he’s come up in the world since; he is now an Esteemed Academy Award Winner. Yet going back to discover where the characters from “Trainspotting” landed was, in theory, an inspired idea, because if there’s anything the pop-culture world could use today, it’s an uncut dose of what was great about the ’90s — an era when the audacity and decadent daring of a movie like “Trainspotting” could get right inside an audience’s bloodstream.

Yet “T2: Trainspotting” is not that movie. As fans have discovered by now, it’s a weary tale of downtrodden, no-expectations middle age. In the opening scene, Renton is running again, only this time on a gym treadmill, where he collapses and has a minor heart attack. It’s a depressing kickoff, not because Renton suffers a physical trauma or even because he’s apparently clean and sober. It’s depressing because the movie can’t come up with anything better to symbolize his new state of being than that ancient cliché: the yuppie workout. Renton, we learn, is married but getting a divorce, and since his wife owns their apartment in Amsterdam, he has nowhere to go. So he drifts back to Edinburgh to look up his old friends: Spud (Bremner), still a junkie, now teetering on the edge of suicide (Renton saving him from the barf-blasted baggie in which he tries to suffocate himself is one of the film’s few joltingly funny moments), and Sick Boy (Jonny Lee Miller), now a pimp with one dominatrix who uses her to rig up voyeur-cam blackmail schemes.

All three are up to not very much, and “T2: Trainspotting” is one of those sequels in which the characters spend way too much time lounging around recalling the old days, because that’s really a way of recalling the first movie. The entire film, it turns out, is organized around the sixteen thousand pounds in drug money that Renton made off with at the end of “Trainspotting” (slipping four thousand of it to Spud in a locker). It was a betrayal, and a death dare, and Robert Carlyle’s Begbie, who has sprung himself out of prison, wants revenge. In the first movie, Carlyle had a disarming sleekness that made his brogue-ish jibber-jabber and bloody sloshed machismo all the more shocking; you had to keep reminding yourself that smashing beer mugs into people’s faces wasn’t normal behavior (though apparently in Scotland it is). But Carlyle is doughier now, and Begbie’s rage no longer has that maniacal edge. He’s just an angry bloke with a grudge, which makes the plot of “T2: Trainspotting” about as interesting as that of an “Expendables” sequel.

It’s a dismally mediocre movie, a portrait of dead-end middle age in which the characters fail to enthrall us because they seem so unenthralled with themselves. Of course, you could argue (as a handful of critics have) that there’s something tragic and true in that: that Boyle and company have made a sequel about the scary passing of youth and the way that indolence can take over. But “T2: Trainspotting” is a classic demonstration of the imitative fallacy — i.e., a film about glum, bone-tired middle age shouldn’t be glum and bone-tired. Besides, when you make a sequel to a beloved movie, I don’t care whether it’s “The Godfather” or “Star Wars” or “Toy Story” or “The Fast and the Furious,” you owe the audience a second dose of what it was that made the original film great. “Trainspotting” was about the crazy glory of addiction, and that’s what’s missing from “T2.” It has no addictive fever at all. It’s more like a film that tries to do penance by putting the first movie in rehab.

Much of the film was taken from the Irvine Welsh novel “Porno” (his follow-up to “Trainspotting”), so we’re supposed to say: It’s just honoring its sacred source! But Danny Boyle is a powerful filmmaker who could have shaped this movie any way he wanted. And what he should have done is to take the characters back to a place of dark and dangerous pleasure. After all, it’s not as if addiction is now the province of the young. Thanks, in part, to the momentous rise of the pharmaceutical drug industry, it’s doubtful that the world has ever seen so many middle-aged drug addicts. Getting hooked — on opioids, meth, heroin — out of desperation and poverty and boredom has become the underground religion of our time. And if any movie under the black hole sun was going to give expression to that phenomenon, it could and should have been the sequel to “Trainspotting.”

Maybe that’s the difference between the ’90s and the decade we’re in now. So much of the allure of that earlier time is that it could afford to be carefree about the demons that ail us all. The current era is much more guarded, because it feels like there’s more to lose. But that shouldn’t be an excuse for overly cautious filmmaking; if anything, it’s art that can help to lead us back to a place of greater freedom. “T2: Trainspotting” is a movie about aging bad boys in which the only overdose on-screen is one of stifling do-right correctness.