After a decade of fans demanding Marvel make a film with a lasting impact on the universe, the third Avengers film, “Infinity War,” looks viewers dead in the eye and says one thing: Be careful what you wish for.

Directors Joe and Anthony Russo, who made the sublime Captain America sequels, “Winter Soldier” and “Civil War,” return with a vengeance, taking the Marvel Cinematic Universe into unexplored, emotionally devastating territory. “Avengers: Infinity War” is this franchise’s darkest, boldest and best film yet, and though it won’t make converts out of the superhero-cynical, it rewards long-time fans with the perfect justification for a cinematic universe.

In a normal review, we would provide a brief plot summary at this point, but the odds are, if you’re reading this, you are familiar with the way an “Avengers” movie works — a Big Bad appears and characters from all of the Marvel films have to team up to stop it. In “Infinity War,” the Big Bad takes shape in Josh Brolin’s incredibly intimidating Thanos — a villain who has been teased since the post-credits scene in 2012’s “The Avengers.”

As the promised bad guy to end all bad guys approaches, all corners of the Marvel Universe pull together — from traditional Avengers characters such as Captain America and Iron Man, to newcomers such as Black Panther and Doctor Strange and even the space-hopping Guardians of the Galaxy — resulting in a potentially-overstuffed roster that totals 27 heroes.

The script, written by “Captain America” veterans Christopher Markus and Stephen McFeely, is a masterclass in event film structure. As “Infinity War” was announced as the first of two parts, it could easily have felt like half of a story, but the picture flows perfectly on its own. Though it does have an absolute shocker of a cliffhanger, it also has a narratively-sound three act structure. Markus and McFeely had their work cut out for them but deliver on all fronts, balancing the dark tonal shift with levity.

The film miraculously milks its two and a half-hour runtime for all it’s worth, jumping back and forth between the massive cast of Avengers. Every character in the film gets their time to shine, leading to the first Marvel movie that truly feels like an ensemble. Where the first two Avengers films focused on Robert Downey Jr.’s Tony Stark and Chris Evans’ Steve Rogers, “Infinity War” doesn’t have a main hero. Though some faces get a bit more screen time than others, the film never leans too heavily in one direction.

The closest this film has to a main character is Thanos. The Mad Titan, who has never been in a film for more than a handful of minutes, takes center stage as the most terrifying villain Marvel has ever created. He will make you afraid for the life of your favorite hero every time he’s on the screen. Brolin breathes fantastic life into his motion capture performance, showing the audience a proud, vengeful, all-powerful being who actually has a surprising amount of depth. Although he doesn’t quite measure up to Michael B. Jordan’s spectacular Killmonger in “Black Panther,” Thanos is in the top tier of Marvel villains — he has goals, beliefs and a clearly defined set of morals twisted by his own mind.

“Avengers: Infinity War” is long, packed full of characters and requires knowledge of a decade of films to fully appreciate. If the MCU isn’t up your alley, this movie will be a slog, but for those who love every element, it’s the perfect culmination of one of cinema’s most historic franchises.

• “Avengers: Infinity War”
• Rating: PG-13
• Runtime: 149 minutes
• Score: 5/5 stars