This review is spoiler-free.
★★★First things first: this film won't make much sense to those who haven't seen the prior Marvel Cinematic Universe (MCU) films yet the filmmakers were under no obligation to catch newcomers up. This truly is a sequel to not only the Avengers films but to the entire MCU which began its series of films with "Iron Man." I'll list the films in the comments section for anyone who wants to catch up.★★★
So how is this film? It is phenomanal. You'll know this film will deliver on its trailers' promised ominous tone as soon as the Marvel Studios logo appears. A few minutes later, a plot surprise manifested itself and it turned out that it was just a foretaste of the sudden reveals, slow-burn shockers, and plot swerves which would arrive.
Yet this film isn't merely gloom and shocks...the plot is nicely crafted to serve as a culmination of the MCU without serving as a 2.5 hour trailer for other films (as some modern blockbusters are wont to do), or sans a heartfelt through line.
This film is firmly rooted upon the foundation of the MCU too so you know there's going to be humor. How much humor is there? It's between the mostly serious "Captain America: The Winter Soldier" and the all out comedy, "Thor: Ragnarok." How is the humor though? It is expertly implemented. It is organic (it's largely based on the realistic reactions of character interactions), and it welcomely relieves the overall tension yet it is thankfully non-present during the harrowing segments of the film.
The action is also appropriately grand yet the battles are surprisingly, and agreeably, kept on a personal scale (the violence is by far the most brutal in any MCU film by the way).
Now has come the time to talk about Thanos, the villain who was teased six years ago in the first Avengers film. He is exceptionally disturbing; a genocidal leader with a savior complex who believes, even in the depths of his heart, that his warpath is morally good. This nuanced character, largely unconcerned with vengance, is expertly portrayed. The rest of the actors do fantastically in their diverse roles as well; drawing viewers in with their optimism and slicing one's heart with their tears.
Unfortunately there were a few hecklers in the audience as if they had delusions that they were watching a movie in their living room...during a loud party (fortunately they restrained their disturbances to a few moments). However, the last 20 minutes, or so, of the movie are so powerful that it clamped the mouths of those attention seekers as the finale's transfixing power pierced and flourished within every soul in the auditorium. This pervasive power continued even in the last few minutes as the film transitioned into a relative hush. I won't say what tone the movie ends on but I will say that the very last shot of the film symbolically cemented what happened in the final moments and I have never seen anything like it. When "Infinity War" ended, I realized that I had never experienced a superhero film with a conclusion such as this and one which would continue to sit within my spirit long after the credits rolled.
The post-credits stinger reinforced the feelings which the ending infused within the crowd yet it provided another aspect ripe for discussion as well.