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It's too much. The nineteenth instalment of the Marvel Cinematic Universe (which doesn't count other Marvel series such as Deadpool, X-Men or the Fantastic Four) is a colossal thumping, thudding mess with too many characters, too wide a scope, too much happening, too many story threads, too large a villain and too high a prize at stake. Dragging in all the original Avengers ensemble (Iron Man, Thor, Captain America, Black Widow et al) as well as more recent additions (Vision, Scarlet Witch, Black Panther), then shoehorning in the Guardians Of The Galaxy gang, it runs for two and a half hours and doesn't reward either your patience or your backside with a satisfying conclusion as it's all acting as mere set-up for an (presumably) even bigger conclusion next year with even more characters.
The other trouble is that it does demand some basic level of familiarity with the MCU line-up: personally I've seen them all once on their theatrical release (except for a few which I caught up with on DVD) but I can't be bothered to marathon them all every time a new one comes out: unlike the Saw or Fast And Furious series it frankly seems too much like homework. There's little point in a synopsis: all you really need to know is that super-evil Thanos (Josh Brolin) is searching for the six Infinity Stones with which he can control time, space, death and reality and fulfil his destiny of slaughtering half the Universe, because nonsense. Thor bumps (literally) into the Guardians Of The Galaxy; Vision and Scarlet Witch have gone off grid, and Black Widow and Captain America have to track them down; Tony Stark and Peter Parker team up with Dr Strange, and the whole thing ends in a huge pitched battle in Wakanda.
Flitting around the world (New York, Scotland, Africa) and assorted alien planets, Avengers: Infinity War alternates the flip, sitcom character banter of the established characters (Stark and Peter Parker, and the GOTG mob, especially) with the now inevitable scenes of destruction and CGI superheroes smacking one another repeatedly through brick walls and/or flying through space. But for all that apocalyptogeddon spectacle and mass carnage, it's completely unengaging and in places quite dull: we've seen it so many times before and this time it's being yelled at us rather than merely shouted. It's not actually explained why Thanos needs the Infinity Stones anyway: to all intents and purposes he's already a god (a cursory Google suggests he's anything up to 2000 years old already), he's more than capable of wiping out half the population wherever he goes, and he's easily able to keep multiple superheroes at bay before he acquires the stones. Even his minions are more than capable of thwacking the heroes aside. Presumably he only wants them because it'll make his great genocidal quest easier.
Most of the characters get their acting or action moment in the spotlight, making the film an impressive juggling act of satisfying each individual's fans; the Black Panther cast get less opportunity, mainly because the action doesn't shift to Wakanda until the third (fourth?) act. In a pleasingly ballsy move, the film actually steps up a gear in its final stretch and kills off more than a couple of regulars, leaving no suggestion that they'll somehow be resurrected for another film or two (although they'll doubtless be recast for all-new standalone reboots in a few years time, because they always do). "Thanos Will Return", claims the end credits crawl, but several much-loved heroes will not. Sure, there's some pleasure to be had, but that's mostly in the character backchat. To be honest, it was more fun when Tom Baker's Doctor Who spent an entire series seeking the six segments of the Key To Time. On this scale, at this cost ($315,000,000 according to Google), with this much stuff going on (no less than twenty four featured characters on the poster artwork and twenty seven star names in the credit block, the order and relative prominence of which must have been a negotiating nightmare), it's just too much.