Thousands of years ago, the Celestials, god-like cosmic entities, created the Eternals, a race of immortal beings, and sent them to Earth to protect humankind from the Deviants, ferocious creatures intent on humanity's destruction. Across the centuries, these Eternals stay hidden, guiding and observing human civilization from a distance while adhering to a strict non-interference policy in human conflict, unless Deviants are involved. The Eternals, each with unique cosmic powers, form a close-knit, albeit sometimes strained, family unit. They believe their mission accomplished when the Deviants are seemingly eradicated and, over time, they disband to live separate lives among humanity.

In the present day, an unexpected incident forces the Eternals to reunite. Now scattered around the globe and assimilated with human society, they must overcome their personal differences and ancient conflicts. The shocking return of the Deviants, seemingly more powerful than before, prompts the Eternals to question the true purpose of their long vigil on Earth and the motives of their creators. The team must gather and confront the new threats, while secrets about their past begin to unravel, challenging their perception of themselves and their duty to Earth's inhabitants.