Twenty-three days after Thanos (Josh Brolin) disintegrated half the life in the universe with the Infinity Gauntlet, Captain Marvel (Brie Larson) rescues Tony Stark (Robert Downey Jr.) and Nebula (Karen Gillan) from their stranded spaceship, and they join the surviving Avengers: Bruce Banner/Hulk (Mark Ruffalo), Steve Rogers/Captain America (Chris Evans), Thor (Chris Hemsworth), Natasha Romanoff/Black Widow (Scarlett Johansson), James Rhodes/War Machine (Don Cheadle), and Rocket (voice of Bradley Cooper). They track down Thanos on an otherwise uninhabited planet, hoping to seize the Infinity Stones to reverse what he has done, but Thanos has destroyed them. In a rage, Thor decapitates him.
Five years later, Scott Lang/Ant-Man (Paul Rudd), escapes from the Quantum Realm, where he was stranded when those able to pull him out died. For him, only five hours have passed. He tells Romanoff and Rogers that they might be able to use the Quantum Realm to go back in time and steal the Infinity Stones before Thanos does. By this time, Stark is married to Pepper (Gwyneth Paltrow) and has a child, so he hesitates to change things, but thinking of Spiderman’s sad death in his arms, he agrees to help. Banner has finally merged with Hulk, blending strength with genius, and helps Stark build a time-machine, though not without arguments, debates on the nature of time-travel from the movies, and jokes about Back to the Future. Banner and Rocket go to New Asgard in Norway to recruit Thor, but find him despondent, alcoholic, and overweight. Black Widow tracks down Hawkeye (Jeremy Renner), who has become a vicious vigilante assassin since his family was wiped out. Both are convinced to help.
Banner, Lang, Rogers, and Stark travel to New York in 2012, where several of the Infinity Stones are located. Banner visits the Sanctum Sanctorum before Doctor Strange (Benedict Cumberbatch) became the Sorceror Supreme and convinces the Ancient One (Tilda Swinton) to give him the Time Stone. At Stark Tower, Captain America breaks in and steals the Mind Stone, though he is forced to defeat his younger self to do it, but Stark and Lang are unable to steal the Space Stone and Loki takes off with it. So Rogers and Stark travel to SHIELD headquarters in 1970, when the stone was held by Tony’s father Howard Stark (John Slattery). There is a tender moment when Howard, whose wife is now pregnant with Tony, share thoughts about family. Rocket and Thor travel to Asgard in 2013 and obtain the Reality Stone from Jane Foster (Natalie Portman), though not before the world-weary Thor gets a pep-talk from his mother (Rene Russo). Also, he gets his hammer back. You can’t touch this. Nebula and Rhodes travel to Morag in 2014 and steal the Power Stone before Peter Quill/Starlord (Chris Pratt) can steal it. Through Nebula’s Cybernetic Implants, Thanos learns what is going on. He captures Nebula and sends 2014 Nebula, still loyal to him, into the future to undo the Avengers’ plans. Barton and Romanoff travel to Vormir in 2014, where the Soul Stone can only be captured with a sacrifice. Black Widow sacrifices herself so Hawkeye can join his resurrected family.
They are reunited in the present. Stark has built a Gauntlet and they mount the stones and trigger the Gauntlet, but evil Nebula uses the time machine to transport Thanos to the present, where he destroys the Avengers’ headquarters and attacks with an overwhelming army of horrid creatures. He is just about to succeed once again, but a revived Doctor Strange arrives with an army of sorcerers, and the restored Avengers and Guardians, the Ravagers, and the armies of Wakanda and Asgard appear to confront him. Captain Marvel destroys Thanos’ warship, but Thanos overpowers her and seizes the Gauntlet. Stark steals the stones and disintegrates Thanos and all his forces, sacrificing his own life in the process. After Stark’s funeral, Thor appoints Valkyrie the ruler of New Asgard and joins the Guardians of the Galaxy. Rogers returns the Infinity Stones to the past but stays there to live with his lost love Peggy Carter (Hayley Atwill).
The movie, directed by the Russo Brothers, began life as Avengers: Infinity War, Part Two, but that was changed when it was obvious that it was more than the second half of the other movie. It is the culmination and big finish of the entire MCU saga, featuring pretty much every actor from 23 movies. The heroes journey back to scenes in every Avengers movie, and the final confrontation is something like a 40-minute battle. Call it the Battle of the Infinite Armies. Alan Silvestri, who composed the scores for Avengers and Avengers: Infinity War—not to mention Back to the Future—created the complex and powerful score, using the themes for Ant-Man, Captain America, Doctor Strange, and Captain Marvel in the process. It is rife with references to time-travel movies. Most of the films in the series have been an adventure, or a lark, or an outright comedy, but this one, though it does have those elements, is a drama. That’s why you cast actors of the caliber of Robert Downey Jr., Chris Hemsworth, Chris Evans, Mark Ruffalo, Karen Gillan, and Scarlet Johansson to play comic-book characters.