In 5000 BC, a group of ten super-powered beings known as the Eternals are dispatched to Earth aboard their starship, the Domo. Led by the wise and spiritual Ajak (Salma Hayek), this diverse team includes Sersi (Gemma Chan), an empathetic Eternal who has a profound bond with humanity, Ikaris (Richard Madden), who possesses the ability to project cosmic energy beams, and Kingo (Kumail Nanjiani), a Bollywood star enamored with fame. Other members include Sprite (Lia McHugh), a childlike Eternal skilled in creating lifelike illusions; Phastos (Brian Tyree Henry), the team's brilliant tech inventor; Makkari (Lauren Ridloff), who can move at superhuman speed; Druig (Barry Keoghan), a manipulator of minds; Gilgamesh (Dong-seok Ma), the strongest of the Eternals; and Thena (Angelina Jolie), a fierce warrior capable of crafting any weapon from cosmic energy.
Their mission, as assigned by the Prime Celestial Arishem (voiced by David Kaye), is to eliminate the Deviants, creatures that threaten intelligent life on Earth. After their last battle with the Deviants in 1521, the team's unity begins to fracture, and they mostly drift apart for the next five centuries.
In 1521, Thena suffers from the Mahd Wy'Ry, causing her to spiral into violent episodes. Ajak diagnoses her condition as a result of her overwhelming memories, proposing a painful option: to wipe her memories clean. Meanwhile, Druig becomes increasingly disheartened by mankind's conflicts and decides to part ways with Ajak's non-interference policy. Gilgamesh chooses to stay by Thena's side, promising not to erase her memories.
Fast forward to modern-day London, where Sersi and Sprite live together. Sersi, having been left by Ikaris long ago, is now in a relationship with a human named Dane Whitman (Kit Harington), who works at the Natural History Museum. Their quiet existence is disrupted when they are ambushed by the Deviant Kro ([Bill Skarsgård]), with Ikaris arriving just in time to fend it off. The Eternals soon realize that the Deviants have returned, now possessing the ability to heal themselves. Their reunion with Ajak is cut short when they discover she has been killed by Kro.
Chosen as Ajak's successor, Sersi gains the ability to communicate with Arishem, propelling her to gather her scattered teammates. Recognizing Kingo's fame, they track down Gilgamesh and Thena in Australia, where Gilgamesh shares Thena's trauma surrounding their home planet's destruction.
Contacting Arishem, Sersi uncovers the true purpose of the Eternals: they were not simply created to battle the Deviants but to ensure Earth is prepared for the "Emergence" — a cataclysmic event tied to the birth of a new Celestial. The stakes grow evident as Arishem reveals that the very existence of Earth depends on the intended birth of the Celestial Tiamut, which would lead to its destruction.
Empowered by their love for humanity, the Eternals come together with a plan to halt the Emergence. They realize that Thena's mental struggles are tied to her memories of past Emergences, and they resolve to put Tiamut to sleep until they can devise a solution. Their journey leads them to Druig, located in the depths of the Amazon rainforest, where they face attack from the Deviants again. Tragically, Gilgamesh sacrifices himself to save Thena during the skirmish.
The team recruits Phastos, who has lost hope in humanity's potential after his technology facilitated grave destruction during World War II. He devises a plan for Druig to amplify his mind-control powers through the Uni-Mind, a collective strength of the Eternals. Their battle reaches a climax as they confront Ikaris, revealing his betrayal; he had fed Ajak to the Deviants due to his unwavering loyalty to Arishem's vision.
As Sersi questions her leadership capabilities, Thena reassures her of her bond with humanity, asserting that Sersi's natural inclination to protect humans makes her the rightful leader. They ultimately intercept the Emergence at an active volcano in the Indian Ocean, where a fierce fight ensues against Ikaris and Kro.
In a twist of fate, Ikaris hesitates to destroy Sersi because of his love for her. By channeling the Uni-Mind's energy, Sersi manages to transform Tiamut to marble, effectively averting catastrophe. Consumed by guilt over his choices, Ikaris flies into the sun, while Sersi grants Sprite's wish to be turned into a human, breaking free of her eternal childhood.
Though Thena, Druig, and Makkari embark on a new adventure aboard the Domo to seek out other Eternals, Sersi, Phastos, and Kingo remain on Earth. As Dane prepares to reveal a significant secret about his lineage, they are forcibly taken into space by Arishem, who expresses his discontent with their actions yet spares humanity's fate, seeking to judge the worthiness of humans through the Eternals' memories.
As the story unfolds, a mid-credits scene introduces Eros (played by Harry Styles), the brother of Thanos, and his sidekick Pip the Troll, who offer their assistance to Thena, Druig, and Makkari. In a post-credits moment, Dane uncovers a mysterious chest from his ancestry, which contains the legendary Ebony Blade, when he is posed with a pivotal question about his readiness for its power.
