Having seized the Power Stone from the planet Xandar, Thanos and his followers—Ebony Maw, Cull Obsidian, Proxima Midnight, and Corvus Glaive—intercept a ship filled with the last remaining survivors of Asgard’s destruction. In a ruthless display of power, Thanos obliterates the vessel and slaughters all surviving Asgardians. He manages to capture Thor, overpower Hulk, and murders Loki all while acquiring the Space Stone from the Tesseract. With the might of the Power Stone behind him, even Hulk is unable to resist Thanos, soon succumbing to fear. Before his demise, Heimdall sends Hulk to Earth using the Bifrost but meets his own end at Thanos’s hand. With his lieutenants, Thanos leaves to destroy the ship completely and instructs them to retrieve two more Infinity Stones from Earth: the Time Stone from Doctor Strange and the Mind Stone from Vision, directing them to meet him on Titan.
Upon crash-landing at the Sanctum Sanctorum in New York City, Hulk reverts back into Bruce Banner and warns Stephen Strange and Wong of Thanos’s diabolical plan to exterminate half of all life in the universe. In a bid to counter this menace, Strange seeks out Tony Stark. Thanos once tried to annihilate life planet by planet, realizing that such a slow approach was ineffective. Now, armed with the Infinity Stones, he believes he can reshape the universe and save existence from itself. While the earlier attack in New York was initiated by Loki, it was orchestrated by Thanos.
The Big Bang birthed six elemental crystals, each exercising control over a fundamental aspect of existence: Time, Space, Reality, Mind, Power, and Soul. Thanos already possesses the Power and Space stones. Wong informs the group that with all six stones, Thanos could manipulate the universe with a mere snap of his fingers. Vision safeguards the Mind Stone, yet he has deliberately shut off his tracker. With the threat of Thanos looming, Rogers must find Vision, despite the estrangement from Stark. Bruce insists that Tony must reconcile with Steve Rogers to face the incoming threat.
Soon, Maw and Obsidian arrive to seize the Time Stone from Strange, catching the attention of Peter Parker. Still struggling with his own failures, Bruce is unable to transform into the Hulk after his brutal defeat by Thanos. Maw captures Strange but fails to obtain the Time Stone due to an enchantment surrounding the amulet that holds the stone. In a daring twist, Stark and Parker fend off Maw by blowing out a section of the ship, sucking Maw into the abyss of open space. As Strange grows anxious about the Time Stone falling into Thanos's grip, Stark proposes that engaging Thanos directly may be the key to victory, even as they continue their pursuit. Strange cautions Stark that he would not hesitate to sacrifice him and Parker if it meant safeguarding the stone.
Meanwhile, Midnight and Glaive ambush Wanda Maximoff and Vision to reclaim the Mind Stone from him. Rogers, Natasha Romanoff, and Sam Wilson come to their aid, retreating to the Avengers Compound where they meet with James Rhodes and Bruce. Vision, ready to sacrifice himself, suggests that Wanda destroy the Mind Stone, but Rogers proposes an expedition to Wakanda, where they might safely extract it without ending Vision’s life.
In a timely intervention, the Guardians of the Galaxy respond to the distress signal from the Asgardian ship and rescue Thor, who had been frozen in the vacuum of space. Upon learning that Gamora is Thanos’s daughter, Thor is shocked to realize the connection. Gamora reveals her wish to thwart her adoptive father's quest for the Reality Stone, safeguarded by the Collector on Knowhere. While she is eager to confront Thanos, Thor opts to journey to Nidavellir to forge a new weapon, wrecked by his sister Hela. Torn between these diverging paths, the group splits into two factions. Gamora carries the painful memory of Thanos’s conquest of her planet, revealing to Quill that she was once taken in when she dared to challenge him. Thanos bestowed upon her a perfectly balanced knife, a dark reminder of their strained bond. She urges Quill to promise her that should Thanos capture her, he must take her life.
Rocket and Groot accompany Thor to Nidavellir, where they discover that Thanos has already ravaged the place, coercing the dwarves into forging his gauntlet, only to eliminate them afterward. Eitri, the chief dwarf, is left alive but tortured, while Thor, Rocket, and Eitri work together to forge a battle-ax called Stormbreaker, which holds the potential to kill Thanos. Thor nearly loses his life as he unleashes the forge's immense power, while Groot supplies the ax's handle. Stormbreaker also possesses the ability to control the Bifrost.
At Knowhere, Quill, Gamora, Drax, and Mantis find Thanos, who is in possession of the Reality Stone. Utilizing the stone's capabilities, Thanos distorts reality, tricking Gamora into believing she has harmed him. In the end, Thanos captures Gamora, expressing his perverse notion that after decimating half of her home planet’s population, the remaining children thrive in a stable environment. With his twisted ideology, Thanos argues that he alone grasps the necessity of his actions to prevent the universe from wasting resources and collapsing under its own weight.
As it unfolds, we learn of Nebula’s capture when she attempts to infiltrate Thanos's stronghold to slay him. After enduring torments by Thanos, Gamora reveals the hiding place of the Soul Stone during their visit to Vormir. Upon arrival, the Red Skull, custodian of the stone, informs Thanos that its acquisition demands the sacrifice of someone he holds dear. With a heavy heart, Thanos plummets Gamora off a cliff, sealed in his tragic quest for power.
Meanwhile, Nebula escapes her confines and rallies the remaining Guardians to converge on Thanos’s barren world, Titan. Stark, Parker, and Strange land there and are met with conflict from Quill, Drax, and Mantis. Upon mutual recognition, they pause their clash and plot to wrest the Infinity Gauntlet from Thanos. With Strange using the Time Stone to explore countless possibilities, he identifies only one scenario where Thanos meets defeat.
As Thanos arrives, he engages in dialogue with Strange, rationalizing his horrifying plans as a necessary evil born from desperation, all fueled by a universe teetering on the brink due to overpopulation. The gathered heroes bravely join forces to restrain him, initially showing promise as they manage to subdue him. However, when Quill learns the truth of Gamora’s fate, he succumbs to rage and attacks Thanos, inadvertently dismantling their hard-fought hold on him. Wielding the Infinity Stones, Thanos bests them, eventually locking eyes with Stark. Mortally wounded yet spared, Stark is left baffled as Strange hands over the Time Stone to Thanos.
In a climactic face-off in Wakanda, Rogers revisits his bond with Bucky Barnes before facing an impending invasion from Thanos’s army. The Avengers, alongside T'Challa and the Wakandan forces, prepare to fend off the impending attack, while Shuri, T'Challa’s sister, diligently works to extract the Mind Stone from Vision. Banner, trapped in Stark's Hulkbuster armor, grapples against the onslaught while Thor, Rocket, and Groot arrive as reinforcements. The tide turns as Midnight, Obsidian, and Glaive fall, yet Thanos swiftly overwhelms the defenders. Despite Maximoff successfully destroying Vision and the Mind Stone, Thanos, wielding the Time Stone, turns back time to restore Vision before ripping the stone from his head, killing him once more.
As Thanos claims the final stone for his gauntlet, he experiences a surge of unparalleled power from all six stones. Delighted yet briefly distracted, Thanos underestimates Thor, who drives Stormbreaker into his chest. With one final jab, Thanos gloatingly tells Thor that he should have aimed for his head. As Thanos raises his gauntlet, reality shifts with a thunderous snap that echoes through the universe, sparking white light.
Thanos finds himself in a serene realm, surrounded by water, within the Soul Stone. There, he encounters a young Gamora, appearing as the innocent child she once was when he ravaged her home. She innocently asks if he fulfilled his destiny, to which he replies, “Yes.” When she inquires about the cost of his success, he solemnly responds, “Everything.”
Back in Wakanda, Thor, burdened with dread as he realizes the consequences of Thanos’s actions, demands to know what has befallen them. In silence, Thanos, gauntlet burnt into his flesh, uses his fading strength to escape with the Space Stone.
In a shocking aftermath, half of all life across the cosmos turns to dust, including Barnes, T'Challa, Groot, Maximoff, and Wilson, all disintegrating before the eyes of their allies on Earth. On Titan, Mantis, Drax, Quill, Strange, and Parker also succumb to this fate. Stranded on Titan, Stark and Nebula alone survive, while Banner, M’Baku, Okoye, Rhodes, Rocket, Rogers, Romanoff, and Thor are left standing on the battlefield of Wakanda.
In the quiet aftermath, Thanos finds peace, gazing at a tranquil sunrise on a distant, idyllic world, his face a mask of quiet satisfaction.
In a chilling post-credits scene, Nick Fury and Maria Hill witness chaos as cars crash and a helicopter crashes into a skyscraper. As Hill fades into nothingness, Fury's last act is to activate a small transmitter, which illuminates a starburst insignia on a vivid red-and-blue backdrop.
