Some sequels are worth the wait.

A full two decades after Danny Boyle’s heroin-soaked “Trainspotting” blew our doors off with its cutting-edge visuals, dark energy and unforgettable soundtrack, not to mention seminal performances from a quartet of talented young actors, the boys are back in town, 20 years older but not necessarily an ounce wiser — and Boyle has delivered that rare follow-up nearly as enthralling as the original.

If you haven’t seen the first one, if at all possible you should spend the next two hours of your life watching it, and then get back to me — and then see “T2.”

It’s not as if “Trainspotting 2” doesn’t work as a stand-alone film, but Boyle regularly peppers in callbacks to the original, from ever-so-brief flashbacks to new scenes that echo some of the classic moments from the original, that the “T2” viewing experience will be vastly more rewarding for those that have seen (and loved) the first movie.

‘TRAINSPOTTING’ SPOILERS FOLLOW

Some 20 years ago, Ewan McGregor’s Mark Renton skipped Edinburgh and his partners in crime and addiction with some 16,000 pounds of drug money that wasn’t all his. Mark left 4,000 pounds for his sad-sack pal Spud (EweN Bremner), but he stiffed the manic and temperamental “Sick Boy” (Jonny Lee Miller) and the even more unbalanced and dangerous Begbie (Robert Carlyle).

Renton has been working as an accountant in Amsterdam, living the straight life, but now he’s ready to face the past and own up to his sins. He comes home, where all three of his former best mates are still living on the fringes of society and still seriously messed up.

Spud is still struggling with hardcore addiction, and has given up on life to the point of contemplating ending it all. Sick Boy has hooked up with an enigmatic 20ish Bulgarian beauty (Anjela Nedyalkova) with more going on upstairs than Sick Boy might realize. He’s running a crummy pub, using any and all available cash to score coke, and scheming to turn the pub into a “classy” brothel.

As for the psychotic Begbie, he has just escaped from prison. His attempts to reconcile with his wife and grown son grow increasingly awkward, desperate and tension-filled. And once Begbie learns Renton is back in town (in one of the funniest sequences of any movie so far this year), he’s all about hunting down the traitor and killing him. (“Trainspotting 2” bills itself as “T2,” as if Boyle and the producers are acknowledging they’re trying to make one memorable sequel  — and Begbie’s pursuit of Renton is only a shade less relentless and untiring than Robert Patrick’s T-1000 in “Terminator 2.”)

Working for a script by John Hodge (who adapts and expands upon author Irvine Welsh’s original creations), Boyle shares our affection for these characters — but constantly reminds us that while they’re fascinating, they’re fascinating disasters. When Renton and Spud, and even to some extent Sick Boy and Begbie, look fondly on the past, they’re conveniently forgetting how pathetic their lives were and how their so-called rebellion against societal norms set them on perhaps permanent paths to utterly worthless lives.

Renton has ostensibly shown the most growth — after all, he kicked the habit, got married and took a respectable job — but in his mid-40s, he’s still flailing his way through life, and even as he tries to make amends with former best mate Sick Boy for one betrayal from years gone by, he eagerly and selfishly pursues another, perhaps even more personal form of double-cross.

Begbie was the one member of the group to avoid addiction — but he’s addicted to the life of crime and the lure of violence, and his attempts to lure his son away from the straight life and his frothing-mad obsession with destroying Renton would be laughable if they weren’t so chilling.

Jonny Lee Miller has a high old time hamming it up as the maniacal, coke-addicted, roughly charming Simon, aka Sick Boy. In fact all four of the main actors do fine work in slipping back into characters they haven’t occupied for 20 years. (“Trainspotting” cast members Kelly MacDonald and Shirley Henderson return in brief but most welcome cameos.)

Renton revisits the “Choose Life” monologue, while the soundtrack revisits a few key entries from the original soundtrack, via remixed versions or covers. “T2 Trainspotting” has one foot firmly planted in nostalgia and the other rooted in the present, and thanks in great part to Boyle’s unique, world-class talent, everything old feels new again, and everything new has the blazing look of an original and blazing piece of art.

★★★★

TriStar Pictures presents a film directed by Danny Boyle and written by John Hodge, based on characters created by Irvine Welsh. Rated R (for drug use, language throughout, strong sexual content, graphic nudity and some violence). Running time: 117 minutes. Opens Friday at local theaters.