I was unprepared for Avengers: Infinity War. And by unprepared, I mean that I went in intentionally ignorant, and that ignorance provoked a response to the conclusion of the film like I was some sort of noob who hasn't seen all of the Marvel movies (I have, including the Hulk). This review contains spoilers!
In some ways, Disney has been planning this for over a decade. Avengers: Infinity War is as much about punctuating a long-running series as it is about reminding us that yes, all these films really do take place in the same universe and not just as a stinger during the credits. It's been a long time coming, and the payoff is huge.
Thanos (James Brolin, being his James Brolin-iest in purple CGI) has come to the conclusion that the Marvel movie cinematic universe is too crowded. You could argue, from a meta-perspective, that this is largely why the Marvel comic series chose to shake things up too -- there are so many characters, so many plot lines, and so many threads to follow that if you're new to the series, it becomes almost impossible to find an entry point without a significant amount of effort. Many reviews have rightly pointed out that much of Infinity War doesn't make sense if you haven't been watching any of the other films, and that's both part of Disney's blessing and curse in what they've managed to create. Well, that's over, because the universe is about to be "cleaned up," and Thanos has decided he's the one to do it.
What that means, exactly, depends on your perspective. Thanos thinks that overpopulation is a serious problem. This is a significant departure from his romance with the literal embodiment of death in the Marvel universe, but a easier to relate to on a planetary scale. It becomes a bit more problematic when you consider a universe scale: it implies Thanos believes the universe and its resources are finite. His plan isn't, for example, to forcibly relocate populations, it's to annihilate half of the planet in an attempt to form a sort of reset for the universe. To that end, Thanos must recover the six infinity stones: The Space Stone (blue), the Reality Stone (red), the Power Stone (purple), the Mind Stone (yellow), the Time Stone (green) and the Soul Stone (orange).
As their names imply, each stone gives cosmic powers to the wielder who can survive touching them. The plot of Infinity War revolves around this simple concept: of Earth's heroes (and some who aren't on Earth, like the Guardians of the Galaxy and Thor) are tasked with keeping Thanos from getting all the stones. Entire Marvel movies were positioned around this topic. Here's who had the stones and has it when Thanos comes for them:
* LOKI (Tom Hiddleston): The Space Stone was used as a weapon in Captain America: The First Avenger and later by Loki in The Avengers. After being defeated, Loki stole it back in Thor: Ragnarok.
* VISION (Paul Bettany): The Mind Stone was used by Baron Wolfgang von Strucker in Captain America: The Winter Soldier, who used it to create Quicksilver and Scarlet Witch. It later was used to create Ultron in Avengers: Age of Ultron and, finally, converted Tony Stark's artificial intelligence known as JARVIS into the android known as Vision. Of all the stone-wielders, Vision has the most to lose.
* THE COLLECTOR (Benicio del Toro): The Reality Stone was given to the Collector after the Asgardians took it from the Dark Elf Malekith in Thor: The Dark World.
* NOVA CORPS: Ronan the Accuser tried to use the Power Stone to destroy the planet Xandar, but the Guardians of the Galaxy stopped him in their titular movie. It was entrusted to the Nova Corps for safe-keeping.
* DR. STRANGE (Benedict Cumberbatch): The Time Stone lies in the Eye of Agamotto, an amulet Strange used in Doctor Strange against Dormammu. Like Vision, Strange is torn between the stone being both a target for Thanos (and the inevitable destruction his focus brings) and using it to fight him.
* GAMORA (Zoe Saldana): Gamora knows the location of the Soul Stone, guarded by none other than the Red Skull from Captain America: The First Avenger, who was transported by the Cosmic Cube/Space Stone there as its eternal guardian. Gamora's knowledge, like Strange's amulet and Vision's brain, makes her a target. Unlike the others, Gamora was raised by Thanos who genuinely loves her like a daughter, despite her betrayal.
Thanos begins the movie with the Power Stone in his possession, which means the film is split into five narratives as Thanos pursues the remaining stones. Excluding the Collector (nobody cared much about him anyway), Thanos extracts a deep personal price from every one of his targets, put simply as Loki/Thor (Chris Hemsworth), Vision/Scarlet Witch (Elizabeth Olsen), Dr. Strange/Tony Stark (Robert Downey Jr.), and Gamora/Starlord (Chris Pratt).
Of course, all these characters have their own story arcs. Thor's and Loki's has played out to its conclusion, so their conflict is brief and Thor's path to redemption longer. Vision/Scarlet Witch have been in the periphery, which makes their doomed romance more of a focus but not enough time dedicated to who they actually are -- I still have no idea what Scarlet Witch's powers are in the MCU besides blasting things with red energy. Dr. Strange and Iron Man's relationship is the most fun -- they're both reformed playboys with huge egos, and their banter (Stark insists on calling Strange a "wizard" to comedic effect) is priceless. Then there's Gamora and Quill AKA Starlord.
Of all the superheroes reintegrated into the larger Avengers universe, the Guardians of the Galaxy have the toughest time. Tonally, they're very different, as exemplified by the sudden welcome addition of "Rubberband Man" to the soundtrack heralding their arrival. The more ruthless members fare better (Drax doesn't seem to notice he's in someone else's movie). Up to this point, Peter Quill/Starlord has emerged mostly unscathed from anything that might challenge his character growth. As the resident Marvel fratboy, Quill's natural humor is crushed when Gamora begs him to kill her if Thanos captures her. The entirety of Infinity War -- and the lives of countless billions -- pivots on this point: Is one relationship worth the lives of the entire universe? Or can Quill stop being a doofus for once and take things seriously? This is a decision Stark has to make when bringing along Peter Parker AKA Spider-Man (Tom Holland). It's a choice Scarlet Witch has to make when extracting the stone from Vision. And it's the challenge Strange has to consider when deciding if Stark's life is worth saving.
In the end, only Strange sees what's coming. I certainly didn't, and I won't spoil it here. But suffice it to say, the cost is so great that it strains audience expectations that the outcome will actually stick -- Disney's not even sharing the title of the next film, so that's saying something -- but for those of us who were blissfully ignorant, what comes next is a punch in the gut.
"Dr. Strange: I went forward in time, to view alternate futures. To see all the possible outcomes of the coming conflict.
Stark: How many did you see?
Dr. Strange: 14,000,605.
Stark: How many did we win?
Dr. Strange: 1."
That better be the sequel to Avengers: Infinity War or I'm going to be pissed.