Avengers: Infinity War is a 2018 superhero film and the nineteenth film entry of the Marvel Cinematic Universe, as well as the third Avengers film overall. It is directed by Joe & Anthony Russo, and written by Christopher Markus & Stephen McFeely — the team behind the second and third Captain America films. This team would go on to complete this movie's sequel, which concludes the story arc started by Infinity War.
After years of planning behind the scenes, Galactic Conqueror Thanos at last comes to the forefront and begins to assemble the six Infinity Stones into the Infinity Gauntlet — an Artifact of Doom with the power to make its wearer omnipotent — in order to enact his plan to save the Universe for generations by eliminating half of all sentient life in an instant. Now the heroes of Earth and the galaxy at large must unite, before untold trillions are killed with the snap of Thanos's gold-plated fingers.
The film (and its successor) collectively serve as the culmination of everything that has happened in the Marvel Cinematic Universe up until the end of Phase 3, filmed back-to-back (with assumed reshoots) between 2017 and 2018. As such, the cast for these movies is quite substantial.