In 2018, a few weeks after the devastating events caused by Thanos, a group of surviving Avengers, including Bruce Banner, Steve Rogers, Thor, Natasha Romanoff, and James Rhodes, plan to use the Infinity Stones to undo Thanos's actions. However, they discover that Thanos has already destroyed the Stones, preventing any reversal. Thor, in his anger, takes matters into his own hands.

Five years later, Scott Lang proposes a potential solution involving time travel. The team, along with Tony Stark and Rocket, decides to retrieve the Stones from the past to change the present. They build a time machine, and the team splits into groups to different points in time to collect the Stones.

Throughout their time-traveling mission, they encounter challenges and revisit key moments in the Marvel Cinematic Universe, interacting with past versions of characters. Along the way, they recruit an overweight and despondent Thor, and Clint Barton, who became a vigilante after losing his family.

As they collect the Stones, including encounters in New York City in 2012 and Asgard in 2013, they face emotional and physical challenges. Romanoff makes a sacrifice to obtain the Soul Stone, and the team faces unexpected complications as they attempt to undo Thanos's actions.

Upon reuniting in the present, the Avengers create a gauntlet to wield the Stones and undo Thanos's disintegrations. However, a version of Nebula from the past, loyal to 2014-Thanos, uses the time machine to bring Thanos and his army to the present. A massive battle ensues, involving the Avengers, Guardians of the Galaxy, and other allies against Thanos and his forces.