The first thought after seeing “Avengers: Infinity War,” the superhero jam session bringing together the entire Marvel Cinematic Universe, is one of cinematic awe.

It’s quite a feat for directors Anthony and Joe Russo to bring together a couple dozen of Marvel’s biggest superheroes — Iron Man, Captain America, Spider-Man, Hulk, Thor, Black Widow, Scarlet Witch, Vision, Black Panther, Falcon, the Guardians of the Galaxy and a bunch more — into a single epic adventure.

What’s more, they do so in a story — scripted by Christopher Markus and Stephen McFeely, who worked with the Russos on the last two “Captain America” movies — that delivers action, emotion and a dramatic conclusion that will leave fans floored.

Buckle up, folks, because this is the movie Marvel has been building toward for a decade, and it’s astonishing.

What reunites the heroes — some, like Iron Man Tony Stark (Robert Downey Jr.) and Captain America Steve Rogers (Chris Evans), stopped speaking after “Captain America: Civil War” — is the villain of all villains. He’s Thanos, a monstrous purple humanoid (performed through motion-capture by Josh Brolin) with a plan to “bring balance” to the universe by making half of its population disappear.

To meet that end, Thanos must collect six Infinity Stones, some of which Marvel’s characters have encountered in past installments of the franchise. To get them, Thanos’ minions first travel to Earth and then to various planets, wreaking destruction at every turn. At many of these stops, some of our heroes, often in new and fascinating combinations, are there to try to stop him.

Along the way, we get fleeting glimpses of the lives our heroes hoped to live in a world that isn’t being attacked by a galactic terror. That’s how we find Stark and Pepper Potts (Gwyneth Paltrow), Vision (Paul Bettany) and Wanda Maximova, aka Scarlet Witch (Elizabeth Olsen), and “Star-Lord” Peter Quill (Chris Pratt) and his first mate, Gamora (Zoe Saldana), who as Thanos’ adopted daughter holds the key to it all.

The Russos find in their telling of this sprawling story the thing Thanos says he wants most of all: perfect balance. The story brings old-line Marvel characters, like Thor (Chris Hemsworth) and Bruce Banner (Mark Ruffalo), together with more recently introduced heroes, like King T’Challa, alias Black Panther (Chadwick Boseman) and Dr. Stephen Strange (Benedict Cumberbatch). The team-ups are often refreshing and exciting, allowing us to see our old friends in new ways.

And then there’s the ending, a shattering 15 minutes that upends all we think about the Marvel universe and will leave audiences in shock. (Yes, there’s a post-credits scene, and you don’t want to miss it.) The end result is an emotional ride that will make Marvel fans laugh and cry, and deliver a cliffhanger that will —

[The end of that last sentence will appear next year, when the second part of the “Avengers” saga arrives in theaters. Sorry.]

Avengers: Infinity War

★★★1/2

Earth’s mightiest heroes take on a universe-altering threat in this epic adventure that builds on a decade of Marvel amazement.

Where • Theaters everywhere.

When • Opens Friday, April 27.

Rating • PG-13 for intense sequences of sci-fi violence and action throughout, language and some crude references.

Running time • 149 minutes.