Thousands of years ago, Steppenwolf and his army of Parademons attempt to conquer and remake Earth through the combined energies of the 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 world.
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 in the palace. Prince joins Bruce Wayne in his attempt to unite the other metahumans, 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 group after his father Silas is kidnapped, along with several other S.T.A.R. Labs employees, by Steppenwolf who is looking for the box left with mankind.
Meanwhile, Steppenwolf attacks Atlantis to retrieve the second Mother Box, forcing Curry to take action. The team receives intel from Commissioner James Gordon that the Parademons are traveling underground, and conclude that Steppenwolf’s army is 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. Wayne decides to use the Mother Box to resurrect Superman, not only to help them fight Steppenwolf’s army but also to restore hope in humanity. Prince and Curry are hesitant about the idea, but Wayne forms a secret contingency plan in case Superman returns as a nemesis.