When word got out six years ago about the coming of The Lego Movie, a cloud of anxiousness began to form around the project. Those beloved plastic bricks deserved better than a feature-length, weaponised toy advert – and mercifully, miraculously, better than that was what they got, by a ridiculous distance.

But fast-forward to 2014, and news that Warner Animation Group was working on a Batman-themed spin-off dredged up the same old fears. Lightning had admittedly struck once, but a double-branded franchise extension had the tenor of a cash-in. Instead, The Lego Batman Movie turned out to be one of the sharpest, most cine-literate superhero films around, with a gag delivery rate so quick it made your ears flap like the doors of a saloon.

You can probably see where this is going. The Lego Ninjago Movie – that’s Nin-JAH-go, not Ninja-go – is based on the line of Lego sets inspired by martial arts and kaiju monster-vs-robot romps, and is the first installment some of us have felt completely at ease with in advance. Yet the end product – this film, unlike its forerunners, is unmistakably a product – is the all-banging, all-yelling embodiment of your very worst toy-to-film-related fears.

The film is unconnected to the long-running Ninjago animated series – all the better to tie into what’s probably being referred to in Warner Bros boardrooms as the Lego Cinematic Universe – and amounts to Power Rangers redux. In the multicultural megalopolis of Ninjago, whenever trouble rears its head, five colour-coded teens transform into their secret ninja alter egos and repel it in a range of enormous, animal-shaped machines.

Trouble, here, is synonymous with Lord Garmadon (Justin Theroux), a volcano-dwelling despot and the estranged father of high-schooler Lloyd Garmadon (Dave Franco), who’s a social outcast thanks to the ongoing exploits of his totalitarian parent. He also happens to be one of the ninjas, but that’s his secret – at least until the second half arrives, and a father-son reconciliation in the treacherous forest beckons.

Take it from someone who’s been happily steeping in Eastern pop culture since childhood: there’s a wealth of source material out there The Lego Ninjago Movie could have stylishly mined for laughs and excitement, much as Lego Batman did before it. But instead, the film’s nine credited writers opt for a tone that might be best described as “internet humour” – a shrill barrage of non-sequitur jokes, in place of the first two films’ precisely crafted roar-your-socks-off repartee. It even play the cute viral video card: one of the villains is a giant kitten called Meowthra (as in Mothra) who “adorably” swats at skyscrapers with his paws, which is funny for the 15 seconds these clips tend to last online, but not much longer than that.

It also looks miserably bland, often verging on ugly – a sad turn-up for a franchise which has already proven dazzlingly adept at staging Michael Bay-style madcap spectacle on a toy-box scale. Ninjago’s action scenes are a drab muddle, and unlike Lego Batman’s Scuttler, and the original’s array of mighty morphing vans, cranes and trucks, the ninjas’ giant robots never convince as toys you’d actually want to play with.

The construction of the plot, meanwhile, is bamboozlingly inept. A live-action framing story with Jackie Chan (who also voices the ninjas’ sensei, Master Wu) serves no purpose other than to allow its star to perform a low-key stunt or two, while vast tracts of expositional dialogue are foisted on Ben Shephard and Kate Garraway, of all people, who make voice cameos as Lego newscasters. The ninja squad themselves, voiced by on-trend comedy names including Kumail Nanjiani and Fred Armisen, are more or less interchangeable, while Abbi Jacobson’s Nya is the kind of solitary cool girl cliché the earlier Lego films took such care to elegantly subvert and sidestep.

Part of the problem is the mere existence of Disney’s Big Hero 6, which recently covered so much of The Lego Ninjago Movie’s remit – the lonely young protagonist, the team of teenage allies, the detail-crammed east-meets-west cityscapes – with infinitely more sweetness and panache. But the film is just as much of a let-down when set beside its predecessors. The lurch from Lego Batman to this takes the franchise from creative blocks to creative block in one disappointing step.