When Batman and Green Arrow enter a mysterious portal hidden in a Greenland cave, they discover the Downworld—a breathtaking yet perilous realm ruled by mythical beings. But their mission turns disastrous when they’re captured and separated—Batman taken by vampires, Green Arrow ensnared by sirens. As communication cuts off, the Justice League scrambles to send help, unaware that rescuing them might awaken the ancient forces that guard the world’s deepest secrets.