Thousands of years ago, Steppenwolf and his legions of Parademons attempt to take over Earth with the combined energies of three Mother Boxes. They are foiled by a unified army that includes the Olympian Gods, Amazons, Atlanteans, mankind, and the Green Lantern Corps. After repelling Steppenwolf’s army, the Mother Boxes are separated and hidden in locations on the planet. In the present, mankind is in mourning over Superman, whose death triggers the Mother Boxes to reactivate and Steppenwolf’s return to Earth in an effort to regain favor with his master, Darkseid. Steppenwolf aims to gather the artifacts to form “The Unity”, which will destroy Earth’s ecology and terraform it in the image of Steppenwolf’s homeworld.
Steppenwolf retrieves the Mother Box from Themyscira, prompting Queen Hippolyta to warn her daughter Diana of Steppenwolf’s return. Diana joins Bruce Wayne in his attempt to unite other metahumans to their cause, with Wayne going after Arthur Curry and Barry Allen, while Diana tries to locate Victor Stone. Wayne fails to persuade Curry, but manages to recruit an enthusiastic Allen onto the team. Although Diana fails to convince Stone to join, he agrees to help them locate the threat if he discovers their location. Stone later joins the team after his father Silas, along with several other S.T.A.R. Labs employees, is kidnapped by Steppenwolf seeking to acquire the Mother Box from mankind.
Steppenwolf attacks Atlantis to retrieve the next Mother Box, forcing Curry into 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. Facing overwhelming odds against Steppenwolf’s army, Wayne decides to use the Mother Box to resurrect Superman, not only to help them fight off Steppenwolf’s invasion, but also to restore hope to mankind. Diana and Curry are hesitant about the idea, but Wayne forms a secret contingency plan in case Superman returns as a nemesis.