In 5000 BC, ten superpowered Eternals—Ajak, Sersi, Ikaris, Kingo, Sprite, Phastos, Makkari, Druig, Gilgamesh, and Thena—are sent by the Celestial Arishem to Earth on their starship, the Domo, to exterminate the invasive Deviants. The last of the Deviants are killed in 1521, and the group's opinions differ over their continued responsibilities and their relationship with humankind. Over the next five hundred years, they mostly live apart from each other, awaiting Arishem's return.
In the present day, Sersi and Sprite live together in London. After Sersi was left by her partner Ikaris centuries earlier, she is now in a relationship with human Dane Whitman, who works at the Natural History Museum. The trio is attacked by the Deviant Kro, with Ikaris arriving and chasing the creature away. Realizing the Deviants have returned, they travel to South Dakota to reunite with their leader, Ajak, only to find that she had been killed by Kro.
Sersi is posthumously chosen by Ajak as her successor, granting her the ability to communicate with Arishem, who reveals that the mission of the Eternals was not to fight the Deviants but to prepare Earth for the "Emergence". Arishem explains that for millions of years, the Celestials have been planting their seeds on populated planets for new Celestials to be born, with the Deviants sent by them to destroy the apex predators of each planet to ensure the development of intelligent life. However, when the Deviants evolved and began hunting the planets' native populations, the Celestials created the Eternals to hunt them. With the reversal of the Blip,[N 2] Earth has reached the necessary population for the birth of the Celestial Tiamut, which will result in Earth's destruction.
Having learned to love humanity, the Eternals reassemble and decide to prevent the Emergence. They reconvene with Kingo, Thena, and Gilgamesh before heading to Druig's residence in the Amazon rainforest, where they are attacked by Kro and the Deviants. The group successfully kill most of them, only for Kro to kill Gilgamesh before fleeing. After cremating him, they visit Phastos, who proposes for Druig to put Tiamut to sleep by amplifying his mind-control powers with the Uni-Mind, a connection between all the Eternals. The team also finds Makkari, who has been living in the Domo. Ikaris reveals he was informed of the Emergence by Ajak centuries before and, choosing to remain loyal to Arishem, fed Ajak to the Deviants six days prior to prevent her from stopping it, before planting her body back at her house to feign ignorance. Sprite joins Ikaris as he flees due to her unrequited love for him, while Kingo also leaves the group as he does not wish to face Arishem or fight his teammates.
Makkari locates the place of the Emergence at the foot of an active volcano in the Indian Ocean, and the Eternals battle Ikaris and Sprite before being attacked by Kro, who is then killed by Thena. Druig knocks out Sprite after she attempts to kill Sersi, and the remaining Eternals work together to subdue Ikaris while Phastos activates the Uni-Mind. Ikaris breaks free of his restraints and approaches Sersi to kill her, but finds himself unable to do so due to his love for her. As both he and Sprite join the Uni-Mind, Sersi gains enough power to turn Tiamut to marble. Guilt-ridden, Ikaris flies into the Sun, while at Sprite's request, Sersi uses the remaining energy from the Uni-Mind to turn Sprite into a human, ending her permanent childlike state.
Thena, Druig, and Makkari depart on the Domo to find other Eternals, while Sersi, Phastos, Kingo, and Sprite remain on Earth. Dane professes his love for Sersi and is about to reveal a secret about his family history when she, along with Phastos and Kingo, are remotely dragged into space by Arishem. Arishem states he is displeased with their treason, but elects to spare humanity if the Eternals' memories show that humans are worthy of living. Vowing to return for judgment, he disappears into a singularity, taking the trio with him.
In a mid-credits scene, Thena, Makkari, and Druig are visited by the Eternal Eros, Thanos' brother, and his assistant Pip the Troll, who offer their help. In a post-credits scene, Dane opens an old chest inherited from his ancestors that contains the legendary Ebony Blade when an unseen person[N 3] questions him whether he is ready for it.