I do not, in general, believe in the phenomenon of actors being the solitary saving grace of an otherwise disposable movie, but I'll say this: Jeff Bridges is clearly the solitary reason that Seventh Son is an absolute blast of a bad movie, the kind that immediately upon departing, I wanted to start listing the names of everybody I knew that I had to force it on, and not just a slog through generic plotting and insultingly clumsy dialogue. I like to think that what happened is that they got through the first day of rehearsal, and after realising what kind of cod-Medieval RenFaire bullshit he'd gotten himself wrapped up in, Bridges decided "well fuck this", and concluded that if he could not act so well as to make the film good, at least he could act so poorly as to make the film hilarious. I have not yet figured out the proper way to describe the accent he's up to in this film, but it ranks right up alongside Dick Van Dyke's in Mary Poppins in the hallowed annals of Are You Supposed To Be From England? Because You Sound Like A Looney Tune. The closest I can come up with is that it sounds the Dude doing an impression of Woody Allen playing Sir John Falstaff. It's dumbfounding and precious and beautiful, particularly when he uses it to assault such Bizarro World dialogue as "Stone chuckers chuck stone, and cattle rippers rip cattle" or "There are witches that need killing. Fucking witches". The latter line doubling as the most wildly discordant swear word in a period movie in ages; it pleases me to assume that Bridges ad-libbed it.

At any moment when Bridges is onscreen, talking in his unplaceable drunken drawl, Seven Son is a triumph; the best comedy of the young 2015, and apt to remain that way for a long time. When he's not onscreen, things take an awfully precipitous tumble. There was some money thrown at this, no doubt, and you can see it in things like the shockingly good visual effects (not consistently so - there are some nasty, under-rendered and ill-composited beasties throughout), and Dante Ferretti's rich production design, creating a world that is physically plausible with visual hints of its own history and unexplored backstory like no high fantasy since The Lord of the Rings. It's like taking one of those hilariously crappy sword-and-sorcery pictures from the early '80s, and throwing money and resources at it that the genre had never witnessed, and no matter how rough the going gets, there's always something to look at with childlike admiration.

The going gets very rough, though. The story isn't particularly objectionable, just a by-the-numbers Chosen One story about Thomas Ward (Ben Barnes), the seventh son of a seventh son, and thus destined by God and numerology to be the ideal apprentice to Master Gregory (Bridges), the last of the "spooks", a line of holy soldiers tasked with rooting out the dark creatures throughout the land. The need for such an apprentice is dire: the once-a-century blood moon is about to rise, and it has freed from her prison in the bowels of the earth Mother Malkin (Julianne Moore), a powerful witch-queen who immediately sets to gathering her army, in order to use the power of the full blood moon to take over the world. But Thomas is a poor study, and he's far more interested in flirting with the mysterious Alice (Alicia Vikander) than do anything else, not suspecting (and probably not caring) that she's Malkin's niece and spy.

That's boring and overdone, sure, but nothing about that necessarily has to be very bad. That only creeps in because, for as much fun as Bridges is plainly having, and as ambitious and complex as Ferretti's sets go - and what the hell, let's throw Jacqueline West and her inventive Middle Ages BDSM couture for the witches - there's not another human soul involved besides those three who has anything but disinterest in the film they're all making. This is patently obvious with even a slight glance at the cast: given a chewy, flashy villain that should make for some splendid hamminess, Moore can barely be bothered to do more than spit out her overblown lines about being Evil and blood-this and blood-that; the CGI dragon she transforms into has infinitely more personality. Vikander can barely contain her disinterest in Barnes, who mistakes keeping his eyes very wide open for being innocent and overawed. Olivia Williams, in the thanklessly small role as Thomas's mother, puts in some effort to bring human warmth to the proceedings, but the proceedings do not want that, and spit her right back out.

The rot goes much deeper, though, all the way to the screenplay adapted by Charles Leavitt, Steven Knight, and Matt Greenberg from the first in a series of children's novels (there does not seem to be much of a fear that Seventh Son will end up sparking a franchise). In a stripped-down form, it seems perfectly, fine, but the way that the story ends up functioning as an aimless tour through underdeveloped aspects of its world proves to be a bit dispiriting; the effect isn't a Tolkienesque vistas-beyond-vistas feeling in which untold stories lurk behind every rock, but the sense that the writers kept throwing in unnecessary details to keep themselves from being bored. The central - in some ways, film-defining - relationship between Thomas and Alice is painfully underdeveloped, leaving an enormous hole that nothing could possibly fill.

Meanwhile, Sergei Bodrov directs with listless slack that benefits nothing and no-one other than Bridges, Marco Beltrami's score drones on in a generic fantasy mode, and Newton Thomas Sigel shoots everything in a rainy palette that almost gives a sense of dour gravity to the film, except that it lacks the dramatic weight to support that. It's outrageously dumb, reductive, and lazy, more boring than bad - but it is aggressively boring. Aye, Bridges comes along and gives exactly the right perversity to make things fun in a dippy, loopy way, and so the aggregate effect of Seventh Son is not, in fact, boredom. But it should take more than one actor trying to blow a movie up from the inside to make it worth watching.