The old adage that the clothes make the man is, curiously, reaffirmed in Matthew Vaughn’s ‘Teen Bond’ picture, which repeats virtually all the beats of “Alex Rider: Operation Stormbreaker” on a far grander, more stylish scale but accompanies them with a heavy dose of nastiness that gives the mixture a distinctly unpleasant aftertaste. Based on a graphic novel, “Kingsman: The Secret Service” aims to recapture the exuberant mix of comedy and mayhem that distinguished the director’s “Kick-Ass,” but in this case the recipe proves less digestible, despite the excellence of many of the ingredients.

Colin Firth steps effortlessly into the shoes of previous Ian Fleming-like heroes as Harry Hart, a sartorially elegant, ultra-smooth superspy who works for no government but for a secret unaffiliated group, the Knightsmen, who wear their perfect Savile Road suits as the modern equivalent of medieval armor. Harry, aka Galahad, is but one of a “round table” (though it’s actually rectangular) squadron of aristocratic types who foil dastardly plots that transcend the political interests of any single nation; the unit is presided over by Arthur (Michael Caine), who’s forced to uncork the decanter of exceptional port they all share—some in person, some by hologram—whenever one of their members is killed. That sad ceremony comes into play when Lancelot (Jack Davenport) is literally sliced in two trying to rescue a scientist (Mark Hamill) from a clutch of kidnappers—his killer Gazelle (Sofia Boutella), a voluptuous martial arts expert with shiny blades for feet. She’s the right hand of telecommunications billionaire Richmond Valentine (Samuel L. Jackson), a lisping megalomaniac with an aversion to gore who’s willing nonetheless to shed some blood to save mankind—or a selected portion of it, at least—from the planetary damage caused by human-fueled climate change. His plan involves handing out SIM cards that will give users free cellphone and internet service forever, but the cost will actually be very high.

Lancelot’s death initiates the search for a replacement, who’s to be chosen from candidates presented by each Knight through a series of hazardous tests engineered by the Knightsmen’s general factotum Merlin (Mark Strong). Most of the elect are upper-class, snobbish types, but Hart, a man devoted to opening the ranks to plebeians, puts forward Gary ‘Eggsy’ Unwin (Taron Egerton), a brilliant but wayward teen suffering the effects of his widowed mother’s (Samantha Womack) marriage to a thuggish brute (Geoff Bell). Eggsy, we’ve been shown in a prologue, is the son of a fellow of lower station whom Harry had brought into the Knights years earlier, but who was killed in an operation, and Hart now feels obligated to help the lad by rescuing him from police custody after the boy trashes the car of one of his stepfather’s gang. He then exhibits his prowess by taking out the whole gang in single combat.

Astonished at Hart’s abilities, young Eggsy happily goes off to training, where he competes against a bunch of snooty upper-crust fellows, but does bond with one of the female recruits, Roxy (Sophie Cookson). Much time is devoted to the tests, which include near-drowning, free-fall skydiving and even a “Perils of Pauline” encounter with an oncoming train. But the one that eventually proves decisive involves merely caring for a puppy, and it will pit Eggsy and Roxy against one another as finalists.

By that time, however, Valentine’s nefarious plot is ready to move forward, and it’s here that the picture begins a serious turn to the dark side. Evidence leads to a church of right-wing bigots in Kentucky, where Hart goes to discover how it’s connected to the scheme. He finds out, but in a sequence of such stunning violence that it turns the larkish spirit the film’s thus far maintained fairly well into something far more sour. It’s choreographed and shot (by George Richmond) with all the brilliance one would expect of Vaughn, but the tone is off; presumably he was aiming for the sort of impact Kubrick achieved in “A Clockwork Orange,” but instead comes up (or down) with the same kind of pandering-to-the-audience, anti-redneck effect that Kevin Smith offered (with far less technical skill, of course) in “Red State.” And there follows an unpleasant coda that, like the fate of Nicolas Cage’s Big Daddy in “Kick-Ass,” is a dreary downer.

The film recovers somewhat in the last act, a colorful Bond-like episode in which Eggsy, Roxy and Merlin work together to foil Valentine’s dastardly plan. But even here the face-off between Eggsy and Gazelle, while again terrifically staged, is grimmer than it should be, and a final gag designed to end things with a sexy twist is just gratuitous. One’s also left wondering about the picture’s political slant, which shows at least one fairly recognizable world leader getting his comeuppance in a way that might remind you of “Scanners” and suggests some self-serving decisions on his part, while offering up a villain with a supposedly principled agenda who, in fact, prefers the planet to the people on it.

Still, one has to appreciate the dapper, debonair quality Firth brings to Hart, and the way the actor’s been seamlessly sewn into the big action scenes. Jackson has fun with obsessed Valentine, even though the one-note shtick does pale somewhat on repetition. As for Egerton, he’s agreeable enough, though he lacks the charisma that marks real star quality, while Strong shows surprising comic chops as the demanding Merlin. And it’s amusing to watch the Cockney Caine acting the pillar of British aristocracy in a film that takes aim, among all its targets, at the English class structure. The production credits are aces all the way, with Paul Kirby’s colorful production design well caught by Richmond’s camera and the visual effects team headed by Steve Begg adding some nicely old-fashioned touches to the cutting-edge pizzazz.

“Kingsman” has so many good points, in fact, that it’s a pity that it runs down in the second half and its dark turn proves overly jarring. “Kick-Ass” maintained a better balance, and by that standard this time around Vaughn manages only a near-miss.