"Avengers: Infinity War" juggles at least thirty major characters over the course of 156 minutes, and while doing so it escapes feeling bloated, underdeveloped, or simply too busy. The key is in the monumental world-building which has taken place within the Marvel Cinematic Universe over the last eleven years. For viewers who have faithfully followed the collectively sprawling previous eighteen films, they already know the involved players—who they are, where they come from, what has led them to become heroes, and the circumstances which have placed them in their present-day moments of truth. Now, for the first time in some instances, they are interconnecting and joining forces, even as fans have the privilege of already being intimately familiar with all parties. Thus, the filmmakers behind 2016's "Captain America: Civil War," directors Anthony Russo & Joe Russo and screenwriters Christopher Markus & Stephen McFeely, can finally cut to the literal chase while bringing a renewed sense of focus and purpose to the "Avengers" franchise Joss Whedon began with 2012's solid if unspectacular "Marvel's The Avengers" and 2015's clunkily forced "Avengers: Age of Ultron."

Thanos (Josh Brolin), that incorrigible intergalactic tyrant, has two Infinity Stones in his possession. Willing to stop at nothing to retrieve the other four, he sets out across the stars, creating a path of terror and destruction in his wake. With two of the powerful elemental crystals situated on distant planets and the other two on Earth, guarded by Dr. Stephen Strange (Benedict Cumberbatch) and Vision (Paul Bettany) respectively, now is most definitely the time for the disbanded Avengers to put their differences aside and reunite. On hand: Tony Stark/Iron Man (Robert Downey Jr.), Thor (Chris Hemsworth), Steve Rogers/Captain America (Chris Evans), Bruce Banner/The Incredible Hulk (Mark Ruffalo), Natasha Romanoff/Black Widow (Scarlett Johansson), Wanda Maximoff/Scarlet Witch (Elizabeth Olsen), Peter Parker/Spider-Man (Tom Holland), T'Challa/Black Panther (Chadwick Boseman), Bucky Barnes (Sebastian Stan), and Guardians crew Peter Quill/Star-Lord (Chris Pratt), Gamora (Zoe Saldana), Drax (Dave Bautista), Nebula (Karen Gillan), Mantis (Pom Klementieff), Rocket (voice of Bradley Cooper), and a newly rebellious teenage Groot (voice of Vin Diesel). If their perilous mission to intercept Thanos' deadly scavenger hunt fails, it will come at a terrible cost: the near-instantaneous decimation of half the universe's population.

"Avengers: Infinity War" races from earthbound struggles to space battles to life-altering confrontations on alien planets, physically separating many of the superheroes even as they fight for one common goal: to save the galaxy as they know and love it. Juggling such a bountiful ensemble naturally keeps certain characters offscreen for long stretches, and yet the film rarely feels like the result of spotty patchwork. There is a thankful cohesiveness to this narrative, and a newfound urgency deriving from Thanos, a bulky lug of a villain who has made appearances here and there in past MCU films but only in this one finally comes into his own as more than a disposable, CG-created baddie. By delving into his twisted mind and unsparing drive for power—and by getting an invaluable assist from Josh Brolin (2015's "Sicario"), who brings intensity and a bitter gravitas to his motion-capture role—directors Russo achieve what many previous MCU releases have not: feature a despicable but not one-note foe who proves as memorable as the heroic protagonists.

Even as there is seemingly little time for small talk or, for example, the layered moral and political debates found at the heart of "Captain America: Civil War," the humanity of its characters shines through. Faced with losing loved ones, with the courage of sacrifice, and even with Bruce Banner's uncooperative abilities to transform into the Hulk when he most needs the green meanie to come out, these powerful women and men come to realize just how vulnerable and, well, mortal they are. In a picture of wall-to-wall special effects, its emotional validity is not misplaced, hitting bluntly and unexpectedly. And there are some genuinely funny moments of levity, too, two of the best given to Danai Gurira's (2018's "Black Panther") Wakanda warrior Okoye during an otherwise rather sobering third act.

"Avengers: Infinity War" may not be the deepest film in the Marvel Cinematic Universe canon, and by the dark, dark nature of its premise it is not the most fun, either. It is difficult to discuss individual action sequences because (1) the movie deserves to be seen and appreciated without spoilers, and (2) the whole two-and-one-half hours rather blends together into a giant set-piece made up of moments rather than situations. The stake claimed by "Avengers: Infinity War" is in its willingness to defy any and all expectations. There is a ballsy anything-goes quality to what happens, and how, and to who that is worthy of a few gasps and a formidable sense of disbelief. Exceptionally well-constructed and always involving—even, that is, when one or two dismaying plot developments threaten to temporarily depress one's enthusiasm—the film betrays because it absolutely must. In order for any of this to have the necessary impact, the viewer must despise Thanos and hope against hope that all will work itself out for good. It may be tough-going for the Avengers until the fourth installment, though, and the next sequel cannot come soon enough. As Dr. Strange previsions it, there are 14,000,605 possible combinations of how things turn out. When Tony asks how many they win, his response is not exactly hopeful: "One." For Marvel fans preparing to see the grim, thrilling "Avengers: Infinity War" for the first time, only one piece of advice will do: brace yourselves.