Avengers: Infinity War continues from Thor: Ragnarok (2017). The movie starts by showing the fate of the spaceship carrying the last Asgard civilians. A super villain appears, tries to gather all the Infinity Stones and wipes out half of the universe. Dozens of superheroes try to stop him. If you do not know what the Infinity Stones or Asgard are, you have not done your homework, i.e. watch the 18 movies before it.
It is remarkable that a movie that contains so many characters manage to present coherent subplots with different superheroes teaming up in new ways, maintain continuity with all the previous movies, and still provide new visual spectacles of superhero battles. At the end it reminds me of The Empire Strikes Back (1980), although its tone is closer to Harry Potter and the Deathly Hallows – Part 1 (2010) – this is indeed a Part 1 movie after all.
Note: There is one post-credits scene.