The conflicting stories of the DCU are explained as a Multiverse, containing many parallel universes and alternate versions of the characters, with the primary DC continuity referred to as Earth-One. A cosmic being from the beginning of time known as the Monitor catalogues these realities, but he has an evil counterpart, the Anti-Monitor, who comes from an antimatter universe. After an accident with antimatter on one universe, the Anti-Monitor begins destroying many of the realities with a wave of antimatter, planning on becoming sole ruler of all realities.
To combat this, the Monitor recruits heroes from across time and space.
Download and watch the all crossover Episodes in The Order of Supergirl, Batwoman, Arrow and The Flash.