The Titan Thanos has begun his plan to unite the Infinity Stones and wipe out half of the life the universe. His plan involves seizing the Stones from remote planets, the Collector of Knowhere and from Xandar, and the several Stones that have come to rest on Earth. In deep space the Guardians of the Galaxy join forces with Thor to defeat Thanos, whilst on Earth the fractured Avengers have to overcome their differences and unite again to fight his armies.
It's entirely possible that no movie in history has had a build-up like Infinity War. Almost every one of the eighteen preceding movies in the Marvel Cinematic Universe has been laying pipe and groundwork for this film, from introducing the Infinity Stones one-by-one to brief appearances by Thanos to the introduction of both the extravagant space opera and mystical sides of the universe through Guardians of the Galaxy and Doctor Strange. Marvel and Disney have shown tremendous restraint and forbearance in not pulling the triggers on those stories too early and making sure they have their ducks lined up in just the right row before finally committing.
Infinity War is an insanely massive movie. Starting as it means to go on - with a massacre which leaves several established characters dead and one MIA (which weirdly goes unmentioned for the whole movie) - the film barely lets up. Characters big and small going right back to the start of the MCU ten years ago (including some you thought you'd never see again) show up, some with large roles to play, some for an extended cameo. Despite the weight of the massive cast, directors Anthony and Joe Russo and writers Christopher Markus and Stephen McFeely somehow create a very coherent film with four distinct acts and the kind of tension filled, multi-pronged final battle on two separate fronts that we haven't seen since Return of the Jedi.
It also helps that although the movie is filled to the brim with heroes and big personalities, the film keeps its focus firmly on a central quintet. Thanos himself dominates proceedings, Josh Brolin (somehow) investing this big purple dude with some real pathos in scenes where we learn more about his backstory, his family and his homeworld. Gamora (Zoe Saldana) also has a major role to play, her family issues with both Thanos and Nebula proving a key emotional motivation for the film. Thor (Chris Hemsworth) also has a lot of screentime, clearly having feeling annoyed after the events of Thor: Ragnarok and determined to kick someone's backside. Scenes pairing him and Bradley Cooper's Rocket Raccoon (or "Rabbit" as Thor insists) are excellent, and then get better when they join forces with a giant space dwarf played by Peter Dinklage. Dinklage's screentime is limited but extraordinarily effective (he also gets arguably the best line of the movie, but it's a really tough choice). Rounding off the central focus is Tony Stark (Robert Downey Jr.) and Dr. Strange (Benedict Cumberbatch), two extremely different people who prove to be an effective team.
Lots of other characters get their moments in the sun (although Mark Ruffalo's Bruce Banner seems to be reduced to a harbinger of doom whilst he's dealing with, er, "performance" problems as Hulk), although the role of Captain America (Chris Evans) in the movie is surprisingly small. The directors know how to deliver a great superhero arrival scene just as all hope seems lost and also how to frame an action sequence. There's a lot of explosions, CG people being flung around and strange creatures and it all flows mostly well, with only a couple of moments where CG fatigue threatens to set in. Infinity War is not a movie any sane person can call restrained, but it's a movie that knows when and where to deploy its monstrous resources (adjusted for inflation, Infinity War is the most expensive movie ever made) to maximum effect.
It's also a surprisingly emotional movie. The weakness of films - and the reason we've seen television explode in comparison recently - is that it's very hard to introduce characters, establish motivation, emotionally invest the audience and then deliver a payoff in under two hours. Infinity War is instead able to draw on almost forty hours of previous character development in the MCU, so even when a fairly minor character bites the dust it hurts a little. When more major characters bite it, things get real (and at least some of these characters aren't coming back).
When the movie runs aground is in its ending, which is impossible to talk about without major spoilers. Suffice to say that the Chekhov's Gun maxim is employed by full force in the film and when you walk out of the cinema - especially if you know the significance of the post-credit sequence and what movie will immediately precede Infinity War II next year - you'll probably be able to immediately pen a fairly close outline of what happens. I mean, if they completely wrong-foot us, fair enough, but some of the choices made in the ending are completely nonsensical if you have any knowledge of what's coming and what's not coming down the Marvel production pipe later on.
Another major weakness is that the film undersells its new team of villains, the Black Order (servants of Thanos). Tom Vaughan-Lawlor as Ebony Maw and Carrie Coon as Proxima Midnight are particularly excellent, but both get limited screen time (especially Carrie Coon, one of the best actresses on TV, who is almost unrecognisable).
Finally, Marvel has gone to some lengths to say that Infinity War is a stand-alone movie and it's as-yet untitled sequel next year (which has already been shot) is a movie in its own right and not just the second half of one bigger story. That's quite frankly untrue, and a lot of the more dramatic and emotional moments from Infinity War will live or die depending on what happens in the sequel.
If you can step out of the meta-knowledge, The Avengers: Infinity War (****) is a very effective action movie with lots of solid action scenes, some real dramatic moments of power and a refreshingly ruthless attitude to its cast of massive stars. It lacks the pacing, focus and character interplay of, say, Guardians of the Galaxy or Black Panther (or even the first Avengers), but's in the upper tier of Marvel Cinematic Universe films and in balancing an unprecedentedly vast cast with solid storytelling, it's almost achieves the impossible.
The film is on general release worldwide from today.