Thousands of years ago, the Steppenwolf and his legions of parademons attempted to take over the Earth with the help of the combined energies of the three Mother Cubes. They were thwarted by a combined army that included the Olympian gods, the Amazons, the Atlanteans, humans, and the Green Lantern. After the repulse of the Steppenwolf army, the Mother Cubes were divided and hidden in different places on the planet.
Humanity is currently in mourning for Superman for two years, whose death causes the Mother Cubes to activate and Steppenwolf to return to Earth. Seeking to regain the favor of his master Darkseid, Steppenwolf seeks to gather the Cubes to unite them, resulting in the destruction of the Earth's ecology and its terraformation in the image of Steppenwolf's home world.
Steppenwolf retrieves the Metrine Cube 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. Wayne goes after Arthur Curry and Barry Allen while Diana tries to find Victor Stone. Wayne fails to convince Curry, but manages to recruit an enthusiastic Allen to 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 and other Star Labs employees are kidnapped by Steppenwolf seeking to obtain the Mother Cube, which is in human custody.