Twenty-three days after Thanos used the Infinity Gauntlet to disintegrate half of all life in the universe, Carol Danvers which was captain marvel rescues Tony Stark and Nebula from deep space and returns them to Earth, where they reunite with the remaining Avengers—Bruce Banner, Steve Rogers, Thor, Natasha Romanoff, and James Rhodes—and Rocket. Locating Thanos on an uninhabited planet, they plan to use the Infinity Stones to reverse "the Snap", but Thanos reveals he destroyed the Stones to prevent further use. Enraged, Thor decapitates Thanos.

Five years later, Scott Lang escapes from the quantum realm. At the Avengers compound, he explains to Romanoff and Rogers he experienced five hours, not years, while trapped. Theorizing the quantum realm could allow time travel, they ask Stark to help them obtain the Stones from the past to undo Thanos' actions in the present. Stark refuses, thinking about his wife, Pepper Potts, and daughter, Morgan, but relents after musing on the death of Peter Parker. Stark, Rocket, and Banner, who has since merged his intelligence with the Hulk's strength, build a time machine. Banner notes changing the past does not affect their present; any changes instead create branched alternate realities. He and Rocket visit the Asgardian refugees' new home in Norway—New Asgard—to recruit Thor, now overweight and drinking heavily, despondent over his failure to stop Thanos. In Tokyo, Romanoff recruits Clint Barton, now a vigilante following Thanos' disintegration of his family.

Banner, Lang, Rogers, and Stark travel to New York City in 2012. Banner visits the Sanctum Sanctorum and convinces the Ancient One to give him the Time Stone. At Stark Tower, Rogers retrieves the Mind Stone, but Stark and Lang's attempt to steal the Space Stone fails, allowing 2012 Loki to escape with it. Rogers and Stark travel to S.H.I.E.L.D. headquarters in 1970, where Stark obtains an earlier version of the Space Stone and encounters his father, Howard. Rogers steals Pym Particles from Hank Pym to use in returning to the present, and spies lost love, Peggy Carter. Meanwhile, Rocket and Thor travel to Asgard in 2013, extracting the Reality Stone from Jane Foster and retrieving Thor's hammer, Mjolnir. Nebula and Rhodes travel to Morag in 2014 and steal the Power Stone before Peter Quill can.

Rhodes returns to the present with the Power Stone, but Nebula becomes incapacitated when her cybernetic implants link with those of her past self, allowing 2014 Thanos to learn of his future success and the Avengers' attempts to thwart it. Determined to rebuild the whole universe so no one will remember what he has done, Thanos replaces present-day Nebula with 2014 Nebula. Barton and Romanoff travel to Vormir in 2014, where the Soul Stone's keeper, the Red Skull, reveals it can only be acquired by sacrificing someone they love. Romanoff sacrifices herself, allowing Barton to obtain the Soul Stone.

Reuniting in the present, the Avengers place the Stones into a technological gauntlet Stark, Banner, and Rocket have created. Banner, the most resistant to the Stones' gamma radiation, is chosen to wield the gauntlet and reverses the disintegrations. Meanwhile, in 2014 Nebula uses the time machine to transport 2014 Thanos and his warship to the present, where he attacks the Avengers' compound looking for the Stones. Present Nebula convinces 2014 Gamora to betray Thanos but is unable to convince 2014 Nebula and is forced to kill her. Confronted by Stark, Thor, and a Mjolnir-wielding Rogers, Thanos outmatches them and summons his army from his warship. Stephen Strange arrives with other sorcerers, the restored Avengers and Guardians of the Galaxy, the Ravagers, and the armies of Wakanda and Asgard to fight Thanos and his army. Danvers arrives and destroys Thanos' warship, but Thanos overpowers her and seizes the gauntlet.

Stark steals the Stones and uses them to disintegrate Thanos and his army, at the cost of his own life.

Tony had a peace of mind but was sad as he will not be with his daughter, pepper and friends.

He just wished he didn't make those mistakes in the past things would be different, if only he has a second chance as he said tony died his soul left his body.