There have been eighteen Marvel Universe movies since 2008. These have introduced a large number of characters, i.e. heroes, most of which come together in this movie to fight a single enemy named Thanos. Previous movies hinted at infinity stones being powerful and dangerous, and the need to keep them from evil. It turns out Thanos is getting his hands on these stones and has an agenda that involves wiping out half the life in the galaxy. Why? His world collapsed due to overpopulation, so he takes it upon himself to solve this problem for everybody else. If he gets all six stones he could kill every other person in the galaxy with just a snap of his fingers.
There have been a number of good Marvel movies in the last decade and many of the stories from the films pick up in Avengers Infinity War from where they left off. The action gets started early and almost never lets up. One might think that this would be bad, but the action is done so well and the characters are so good that the movie feels like something truly special on a grand scale. Never have we seen so many different stories and characters woven together so seamlessly. This isn't just a movie, but an event a decade in the making.
We see more of Thanos than we do any individual hero, making him in effect the main character. This is as much his story as it is anybody else's, and he is played wonderfully by Josh Brolin.
With so many actors and effects, it is not surprising that the movie cost $360 million to make. They got their money's worth. Some have compared the film to Star Wars in terms of entertainment value, which is not a bad comparison.