In 2018, Clint Barton, who is under house arrest due to his involvement in the Avengers' Civil War, spends time teaching his daughter Lila archery at their home while his wife Laura and their two sons prepare for a picnic. Suddenly, he witnesses the horrifying event as everyone around him vanishes into ash, leaving him in panic and entirely alone.
Fast forward three weeks, after Thanos has devastatingly wiped out half of the universe's population with a single snap. Tony Stark and Nebula find themselves adrift in space aboard the Benatar, with their fuel running low and near starvation. In a heart-wrenching moment, Stark records a message for his fiancée, Pepper Potts, believing his time is up. Just then, Carol Danvers locates their ship and brings them back to Earth, where Stark reunites with old friends, including Steve Rogers, [Natasha Romanoff], [Bruce Banner], and James Rhodes. Rocket Raccoon joins Nebula as they mourn their fallen comrades.
Stark expresses his frustration towards Rogers, accusing him of disbanding the Avengers, which resulted in their disunion during the conflict with Thanos. He questions why the heroes only act when it's too late, before fainting from exhaustion. Following a lead from Nebula regarding her father's location and a strange energy reading, the heroes embark on a mission into space to confront Thanos and reclaim the Infinity Stones to reverse the destruction he has caused.
Upon discovering a scarred and vulnerable Thanos cooking in solitude, they attempt an ambush, severing the arm that once bore the Infinity Gauntlet, but find it empty. Thanos calmly reveals that he destroyed the Stones himself, wanting to obliterate their temptation. While Rhodes expresses skepticism, Nebula defends her father's honesty. Ultimately, Thor beheads the defeated Titan in frustration, yet they are left grappling with the grim reality that their quest for resurrection has failed.
Five years pass, and the surviving population struggles to cope with the surreal loss. Rogers now leads a support group to help others confront their grief. Meanwhile, Stark and Pepper are married, living a secluded life with their daughter, Morgan. In San Francisco, Scott Lang escapes from the Quantum Realm thanks to a fortuitous accident involving a rat, only to find a city drastically changed by the Snap. Upon discovering an emotional memorial wall, he fears for his daughter's safety, quickly rushing to find her grown and waiting: she's now a teenager.
At the Avengers Compound, Natasha Romanoff has taken command, learning from Rhodes about Barton's troubling downfall after losing his family. Lang's return sparks a revolutionary idea: they could exploit time travel through the Quantum Realm to retrieve the Infinity Stones from various points in the past—which he proposes to the team in desperation. After initially declining out of fear for his daughter's safety, Stark is eventually motivated to assist upon reflecting on his late protégé, Peter Parker. He crafts a time-space locator, while the team works on practical tests for time travel.
Despite initial struggles with their technology, the remaining Avengers regroup, taking on various roles in their quest. Thor, now visibly depressed and overweight, is lured from obscurity. Barton, consumed by vengeance, is reluctantly convinced to help despite warning signs of his current state. They formulate a strategic plan, pinpointing locations from their past to gather the Stones.
Entering an alternate 2012 during the Battle of New York, they face complications as they encounter their past selves, cleverly maneuvering to snatch the Space and Mind Stones. Rogers, in a heartfelt moment, realizes how fleeting time is as he secretly observes Peggy Carter from afar. As they time jump through pivotal moments—including retrieving the Reality Stone and successfully wrestling with their own ghosts from the past—each member faces their trials while grappling with their grief and regrets.
In a tense sequence on Vormir, Barton's clash with Natasha Romanoff leads to a deeply painful sacrifice where she heroically leaps to her death, obtaining the Soul Stone in return. With heavy hearts and newfound resolve, the Avengers use their newly rebuilt gauntlet to bring back those who were lost. However, an unexpected return of Thanos and his forces ignites a climactic battle that tests their combined strength and willpower.
As Thanos gathers his army to destroy Earth entirely, Steve Rogers hears a poignant call for help, bringing reinforcements that include allies from across the universe. The Avengers rally, igniting the ultimate showdown in a desperate struggle for their lives and the fate of their loved ones.
Ultimately, in a staggering twist, Tony Stark manages to turn the tide by transferring the Stones to his suit, dramatically snapping his fingers to obliterate Thanos and his forces, but at the cost of his own life. As he lays dying, the weight of his sacrifice unravels bittersweet reflections for his friends and family.
The aftermath sees the heroes mourning their fallen comrades while honoring Stark's legacy at his emotional funeral. New roles are established, with Thor appointing Valkyrie as the new ruler of New Asgard and passing on his leadership role to join the Guardians. Rogers chooses to return the Stones to their rightful places, ultimately revealing he chose a profound life with Peggy Carter. In a heartfelt parting, he bestows his shield upon Sam Wilson, leaving the now-vacant legacy of Captain America in capable hands.
