Part sequel, part prequel, and all empty-headed fluff, Ol Parker’s “Mamma Mia! Here We Go Again” is so relentlessly colorful and desperate to please that it seems designed to make you overdose on glitz and goofiness. (Even cinematographer Robert Yoeman seems to have been occasionally blinded by the sun and costumes: in one sequence shot on the sea, the lens glare is overwhelming.) Coming a full decade after the original jukebox musical stuffed with trite but popular ABBA songs, this follow-up relies mostly on tunes that are second-rate even by the Swedish group’s standards; as such it might said to inaugurate a new genre, the junkbox musical.

Cobbling together a story to incorporate the undistinguished but bouncy songs, Parker situates things in two different timeframes. In one, set five years after the first movie, Sophie (Amanda Seyfried) is refurbishing the farmhouse that her late mom Donna (Meryl Streep) had transformed into an inn on an Aegean island, and is preparing its grand reopening alongside suave, unflappable manager Fernando (Andy Garcia). One of her three “dads,” her actual stepdad Sam (Pierce Brosnan), is there to help, though still grieving Donna’s demise. But it’s unlikely the other two, Bill (Stellan Skarsgard) and Harry (Colin Firth), will be able to make the party.

To compensate, Donna’s BFFs Tanya (Christine Baranski) and Rosie (Julie Walters), who were her mates in the trio of Donna and the Dynamos, have arrived. Unfortunately, Sophie’s relationship with Sky (Dominic Cooper) has run into problems. Now off in New York on some sort of internship, he’s offered a job that could keep him there. The potential separation is especially problematic since Sophie soon discovers that she’s pregnant—and unlike her mother years before, she knows who the father is.

That “present day” plot dovetails with an extended flashback to the tale of how young Donna (Lily James), just graduated from Oxford, first came to Skopelos and decided to stay, not only meeting the young Harry (Hugh Skinner), Bill (Josh Dylan) and Sam (Jeremy Irvine) in the process but getting pregnant with Sophie. Younger versions of Tanya (Jessica Keenan Wynn) and Rosie (Alexa Davies) figure into this part of the narrative as well.

With the connivance of editor Peter Lambert, the movie shuffles between the two stories, straining for the most heavy-handed connections between them, while Parker simply inserts songs wherever he can, however tenuous the narrative link might be. The first movie’s structure was ramshackle, but this one’s is even worse; it barely holds together long enough to get to the finish line. There—when the grand reopening of the inn finally happens—things turn into a complete free-for-all, with characters popping up out of thin air for big song sequences that look like something out of a Vegas floor show, with everybody dressed in spandex and doing moves out of a disco playbook. Only Firth, mindful of his shortcomings in the terpsichorean department, plays his stiffness for laughs; the others actually give it the old college try. (Brosnan, on the other hand, does attempt to sing once again. Happily, he does so only briefly, and softly.)

And yet it’s the last thirty minutes or so, as garish and lunatic as they are, that finally bring the movie to some sort of life. Not that Parker doesn’t try in the initial ninety minutes. He leads off with a big Oxford-set number to “When I Kissed the Teacher” that immediately goes for broke; but the underlining theme of the song tastes sour in this age of preoccupation with sexual harassment. As the picture rolls on, James and Seyfried and their young colleagues exhibit better voices than most of the cast did in the first movie, but the songs are inferior for the most part, and James tries so hard to be the irrepressible free spirit that she becomes more irritating than charming.

That isn’t to say that both actresses don’t have bright moments, but the biggest production numbers—like James’s duet with Skinner on “Waterloo”—come across as forcedly frenetic and “cute.” There are nonetheless occasional compensations—James’s sea-set song-and-dance with Dylan is engaging enough, a reprise of “Mamma Mia!” featuring James, Wynn and Davies gets the juices flowing, and when Baranski and Walters take center stage with “Angeleyes,” things pop. For the most part, though, the effort expended on the musical numbers in the initial hour-and-a-half fails to pay off.

That changes toward the close, however, when Cher shows up as Sophie’s diva grandmother and has a show-stopping duet with Garcia to “Fernando.” Once again, the song comes out of left field, but though the number is done with a lot of pizzazz (including a fireworks display), it’s also done relatively straight musically, and since the tune is one of ABBA’s catchiest, it works. Parker then moves to his pièce de résistance, a duet between a ghostly Donna and her daughter on “My Love, My Life,” followed by an unabashed series of cinematic curtain calls to introduce the final credits. This final half-hour is the sort of extended sugar high the movie has been aiming for all along.

One supposes that this shiny, totally synthetic bauble of heart-on-sleeve emotion wrapped up in insanely bubbly music will appeal to those who embraced the first trip to ABBA-land, though even they might have a happier time skipping the first ninety minutes and slipping in only for the last thirty. They should, moreover, agree with the rest of us that at least another ten years would be advisable before attempting a third trip to what is now called La Bella Donna.