what is the storyline?
Steppenwolf and his legions of Parademons had attempted to take over the Earth using the combined energies of the three Mother Boxes.
Justice League (film)
Thousands of years previously, Steppenwolf and his legions of Parademons had attempted to take over the Earth using the combined energies of the three Mother Boxes. The attempt was foiled by a unified alliance including the Olympian Gods, Amazons, Atlanteans, mankind, and extraterrestrial beings from other worlds. After Steppenwolf's army was repelled, the Mother Boxes were separated and hidden in different locations. In the present, mankind is still in mourning two years after Superman's death, which has triggered the Mother Boxes' reactivation and Steppenwolf's return to Earth. Steppenwolf aims to regain favor with his master Darkseid by gathering the boxes to form "The Unity", which will destroy Earth's ecology and terraform it in the image of Steppenwolf's homeworld.

Steppenwolf retrieves one Mother Box from Themyscira, prompting Queen Hippolyta to warn her daughter Diana of Steppenwolf's return. Diana joins Bruce Wayne in an attempt to unite other metahumans to their cause: Wayne
whose at the end of justice league trailer Clark Kent was revealed in a final trailer before the release of the film.
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Justice League (film) [SEP] Plot
Thousands of years previously, Steppenwolf and his legions of Parademons had attempted to take over the Earth using the combined energies of the three Mother Boxes. The attempt was foiled by a unified alliance including the Olympian Gods, Amazons, Atlanteans, mankind, and extraterrestrial beings from other worlds. After Steppenwolf's army was repelled, the Mother Boxes were separated and hidden in different locations. In the present, mankind is still in mourning two years after Superman's death, which has triggered the Mother Boxes' reactivation and Steppenwolf's return to Earth. Steppenwolf aims to regain favor with his master Darkseid by gathering the boxes to form "The Unity", which will destroy Earth's ecology and terraform it in the image of Steppenwolf's homeworld.