Pretty much confirmed by the new trailer.
An alliance between humans, amazons and atlanteans repels Steppenwolf's invasion in pre-historic times and three Mother Boxes are left behind and split between them.
On present day, Batman and Wonder Woman are recruiting a team of meta-humans to protect Earth from an extraterrestrial threat. The Flash agrees, while Aquaman and Cyborg decline.
Cyborg's father, Dr. Silas Stone, is kidnapped by Parademons and the humans's Mother Box is stolen. Cyborg then joins Batman, Wonder Woman and Flash as they venture into the Parademons's nest on an abandoned tunnel connecting Metropolis and Gotham City.
Meanwhile, Darkseid's enforcer Steppenwolf arrives on Earth to retrieve the Mother Boxes, having learned about them from Lex Luthor's experiments with Kryptonian technology, and steals the atlanteans' Mother Box, prompting Aquaman to join the heroes.
The heroes fail to prevent Steppenwolf from stealing the Amazons' Mother Box and find out Steppenwolf is planning to use the Mother Box to open a portal and bring an army of Parademons to invade Earth, and has kidnapped scientist so they can revive Superman.
The heroes rescue Superman from the Parademons and revive him with Lois Lane's help, but he's overwhelmed by the experience and flees.
The heroes confront Steppenwolf and his army on their own and are nearly defeated before Superman joins the fight. Cyborg realizes he's connected to the Mother Boxes and manages to reverse the portal to send the Parademon army back to Apokolips. The heroes then defeat Steppenwolf and send him back as well.
For his failure, Steppenwolf is executed by Darkseid, who vows revenge against Earth. Lex Luthor arranges to be released from Arkham Asylum and also plans revenge. The heroes are embraced by the public and decide to keep fighting together as the Justice League.
