Thousands of years ago, Steppenwolf and his army of Parademons attempt to conquer Earth through the combined energies of three Mother Boxes. They are foiled by a unified army that included the Olympian Gods, Amazons, Atlanteans, humans, and Green Lanterns. After repelling Steppenwolf’s army, the Mother Boxes are separated and hidden in locations around the planet.
In the present, the world is in mourning after the death of Superman. This triggers the Mother Boxes to reactivate and Steppenwolf’s return to Earth in an effort to regain favor with his master, Darkseid. Steppenwolf manages to retrieve the Mother Box from Themyscira, prompting Queen Hippolyta to warn her daughter Diana Prince of Steppenwolf’s return by lighting the invasion fire. Prince joins Bruce Wayne in his attempt to unite other metahumans to their cause, with Wayne going after Arthur Curry and Barry Allen, while Prince tries to locate Victor Stone. The two of them fail to persuade Curry and Stone, but manage to recruit an enthusiastic Allen onto the team. Stone later joins the team after his father Silas is kidnapped, along with several other S.T.A.R. Labs employees, by Steppenwolf seeking to acquire the Mother Box from them.
Meanwhile, Steppenwolf attacks Atlantis to retrieve the next Mother Box, forcing Curry to take action. The team receives intel from Commissioner James Gordon leading them to Steppenwolf’s army, based in an abandoned facility under Gotham Harbor. Although the group manages to rescue the kidnapped employees, the facility is flooded during combat, which traps the team until Curry helps delay the flood so they can escape. Stone retrieves the last Mother Box, which he had hidden, for the group to analyze. Stone reveals that his father used the Mother Box to rebuild Stone’s body after an accident almost cost him his life. Wayne decides to use the Mother Box to resurrect Superman, not only to help them fight off Steppenwolf’s army, but also to restore hope to mankind. Prince and Curry are hesitant about the idea, but Wayne forms a secret contingency plan in case Superman returns as a nemesis.
Clark Kent’s body is exhumed and placed in the incubation waters of the Kryptonian ship alongside the Mother Box, which in turn activates and successfully resurrects Superman. However, Superman’s memories have not returned, and he attacks the group after Stone accidentally launches a projectile at him. On the verge of being killed by Superman, Batman enacts his contingency plan: Lois Lane. Superman calms down and leaves with Lois to his family home in Smallville, where he reflects and tries to recover his memories. With the final Mother Box unguarded, Steppenwolf retrieves it with ease.
Without Superman to aid them, the five heroes travel to a village in Russia where Steppenwolf plans to unite the boxes once again and reshape the world. The team fights their way through the Parademons to reach Steppenwolf, although they are unable to distract him enough for Stone to separate the boxes. Superman arrives and assists Allen in evacuating the city, as well as Stone in separating the Mother Boxes. The group manages to defeat Steppenwolf, who, overcome with fear, is attacked by his own Parademons before they all teleport away.
In the aftermath of the battle, Wayne and Prince agree to rebuild Wayne Manor as a base of operations for the team, with additional space for more potential heroes. Prince steps back into the public spotlight as a hero; Allen acquires a job in Central City’s police department, impressing his father; Stone continues to explore and enhance his abilities with his father in S.T.A.R. Labs; Curry returns to Atlantis; and Superman resumes his life as reporter Clark Kent. In the post-credits scene, Lex Luthor has escaped from Arkham Asylum and recruits Slade Wilson to form their own league.