In 5000 B.C., ten immortal beings known as the Eternals arrive on Earth aboard the Domo, a massive spacecraft sent by the Celestial Arishem the Judge. Their mission is to protect humanity from monstrous predators called Deviants, creatures created to eliminate apex predators so intelligent life could flourish. Led by the wise and compassionate Ajak, the Eternals consist of Sersi, Ikaris, Kingo, Sprite, Thena, Gilgamesh, Druig, Phastos, Makkari, and Ajak herself. Though they possess extraordinary cosmic abilities, Arishem forbids them from interfering in human wars and conflicts, allowing humanity to evolve naturally. Over thousands of years, the Eternals guide civilizations from the shadows while battling Deviants across Mesopotamia, Babylon, and other ancient societies. During their centuries together, Sersi and Ikaris gradually fall in love and eventually marry.
As humanity advances, tensions begin to form within the group. In 1521, during the Spanish conquest of the Aztec Empire, the Eternals battle what appears to be the final group of Deviants. During the fight, Thena suffers another episode of Mahd Wy’ry, a condition causing her to lose control and attack her allies because of fractured memories buried deep within her mind. Gilgamesh subdues her and volunteers to care for her permanently rather than allow Ajak to erase her memories. At the same time, Druig grows furious that the Eternals continue standing by while humans slaughter one another in war. Unable to accept their inaction any longer, he abandons the group to use his mind-control powers to stop human violence himself. Believing the Deviants extinct, Ajak disbands the Eternals and encourages them to live separate lives among humanity. Over the centuries, the group drifts apart, and Ikaris eventually leaves Sersi without explanation, breaking her heart.
In the present day, Sersi lives in London with Sprite and works as a museum curator. She has also begun dating Dane Whitman, a kind but ordinary human unaware of her true nature. One night, while leaving a party, Sersi, Sprite, and Dane are suddenly attacked by a Deviant, shocking the Eternals because the species was believed to be extinct for centuries. Ikaris unexpectedly arrives and helps defeat the creature, though they discover the Deviant is capable of healing itself and evolving. Realizing something is terribly wrong, Sersi, Sprite, and Ikaris travel to South Dakota to seek guidance from Ajak, only to discover her dead inside her home after being attacked by Deviants days earlier.
As Sersi mourns Ajak’s death, Ajak’s glowing communication sphere transfers to her, making Sersi the new Prime Eternal and allowing her to communicate directly with Arishem. Determined to stop the returning Deviants, the trio begins reuniting the scattered Eternals. They first visit Kingo, who has reinvented himself as a famous Bollywood movie star and now travels with his loyal valet Karun Patel, who enthusiastically documents their journey. Next, they locate Gilgamesh and Thena living in isolation in Australia. There, Thena’s Mahd Wy’ry worsens after another Deviant attack. While examining drawings Thena created during her episodes, Sersi accidentally contacts Arishem through the sphere and learns a horrifying truth about the Eternals’ purpose.
Arishem reveals that Olympia, the Eternals’ supposed homeworld, never existed. Instead, the Eternals are synthetic beings created by Celestials in the World Forge. He explains that the Deviants were originally engineered to eliminate predators and help intelligent life evolve, but the creatures eventually evolved beyond their programming and became threats themselves. The Eternals were then created to destroy the Deviants and ensure intelligent civilizations thrive. However, Earth’s population growth serves a hidden purpose: the planet contains a dormant Celestial named Tiamut, whose birth—known as the Emergence—will destroy Earth once enough intelligent life exists to provide the necessary energy. The death of the planet will allow Tiamut to be born and create new galaxies and life across the universe. After each Emergence, the Eternals’ memories are erased before they are sent to another world. Horrified, Sersi realizes Thena’s Mahd Wy’ry was caused by fragments of memories surviving a previous memory wipe after another planet’s destruction.
Refusing to allow Earth to die, the Eternals seek out Druig in the Amazon rainforest, hoping his mind-control powers can place Tiamut back to sleep. That night, they are ambushed by evolved Deviants led by Kro, a powerful Deviant capable of absorbing Eternal powers and memories. During the battle, Kro kills Gilgamesh and absorbs his strength and memories, evolving into a more intelligent humanoid form capable of speech. Before dying, Gilgamesh helps Thena regain control of herself and overcome her fractured memories. The Eternals mourn Gilgamesh’s death in an emotional funeral ceremony before continuing their mission.
The group then travels to Chicago to recruit Phastos, who abandoned humanity after witnessing the devastation of Hiroshima, believing his inventions only accelerated human violence. However, he has since found hope through his husband Ben and their son Jack. Persuaded to fight for humanity’s future, Phastos rejoins the Eternals. They later recover the Domo from beneath the sands of Iraq and reunite with Makkari, who has spent centuries studying humanity’s history aboard the ship. Phastos proposes creating a Uni-Mind, a cosmic connection that would combine the Eternals’ powers and allow Druig to control Tiamut long enough to stop the Emergence.
However, Ikaris strongly opposes the plan. Through flashbacks, it is revealed that Ajak secretly informed Ikaris centuries earlier about the Eternals’ true mission. Although Ajak eventually changed her mind after witnessing humanity’s resilience following the Avengers’ reversal of the Snap, Ikaris remained loyal to Arishem’s design. Six days earlier, he betrayed Ajak by bringing her to a frozen region in Alaska where Deviants had resurfaced due to the planet’s instability during the Emergence. He allowed Kro and the Deviants to attack her, leading to her death. Afterward, he returned her body home and concealed the truth from the others.
As the Emergence begins, earthquakes and natural disasters spread across Earth. Makkari locates the emergence point at a volcanic island in the Indian Ocean. Druig deduces Ikaris was responsible for Ajak’s death, and Sersi learns he has known the truth about their purpose for centuries. Ikaris argues that sacrificing Earth is necessary for the continuation of life across the universe and that Arishem’s design cannot be defied. Kingo sympathizes with Ikaris’ belief in duty but refuses to fight his friends, choosing to leave the conflict entirely alongside Karun. Sprite, who has secretly loved Ikaris for centuries and resents being trapped forever in the body of a child, joins Ikaris instead.
Before departing for battle, Phastos uses Ajak’s communication sphere to create special bracelets that help the Eternals form the Uni-Mind. On the volcanic island, Druig attempts to connect with Tiamut through the Uni-Mind, but Ikaris attacks and incapacitates him before he can succeed. As Tiamut begins emerging from Earth’s crust, massive stone structures rise from the ocean while global destruction intensifies. Phastos, Makkari, and Thena battle Ikaris to protect Sersi, believing her matter-transmutation powers are their last hope. Meanwhile, Kro attacks the others, hoping to absorb additional Eternal powers. Inside a cave, Thena confronts Kro alone and ultimately kills him after resisting his attempts to manipulate her using Gilgamesh’s memories.
During the battle, Sprite creates an illusion of Ajak to distract Sersi before stabbing her out of jealousy and desperation. However, Druig recovers and stops Sprite before she can continue. Phastos briefly restrains Ikaris using advanced technology, but Ikaris eventually breaks free and flies toward Sersi to stop her from killing Tiamut. Yet when he finally confronts her, he cannot bring himself to harm the woman he still loves. Overcome with guilt and memories of their life together, Ikaris surrenders.
At the critical moment, the Uni-Mind unexpectedly activates fully, linking all remaining Eternals—including Ikaris and Sprite—and channeling enough cosmic energy into Sersi. Using her powers, Sersi transforms the emerging Celestial Tiamut into marble before it can fully awaken, saving Earth from destruction. Realizing the enormity of his betrayal and unable to live with his guilt, Ikaris apologizes to Sersi before flying into the Sun and killing himself.
After the battle, Sersi uses the remaining Uni-Mind energy to transform Sprite into a human, finally allowing her to age and live a normal life. Two weeks later, Thena, Druig, and Makkari depart aboard the repaired Domo to search the universe for other Eternals and reveal the truth about Arishem’s lies. In London, Sprite leaves to begin attending school as a human while Sersi resumes her relationship with Dane Whitman.
However, Arishem suddenly appears in Earth’s orbit and abducts Sersi, Phastos, and Kingo for defying his design and stopping the Emergence. He warns that he will study their memories before deciding whether humanity deserves to survive. Elsewhere, Dane prepares to reveal a powerful family secret involving an ancient enchanted sword before being interrupted by a mysterious voice. Days later aboard the Domo, Thena, Druig, and Makkari are approached by Eros—also known as Starfox, brother of Thanos—and his companion Pip the Troll, who claim they know how to rescue their captured allies.
