The coastal town of Bristol Cove is known for its legend of once being home to mermaids. When the arrival of a mysterious girl proves this folklore all too true, the battle between man and sea takes a very vicious turn as these predatory beings return to reclaim their right to the ocean.

Megan Quinn, an aspiring architect, faces multiple tasks – developing her hometown lake into a commercialized entertainment center, forcing Mrs. Frances Figgins, her childhood mentor, out of her family’s home, and helping out at her family café after her father breaks his leg, all while also dealing with the return of her childhood sweetheart, Joe, who broke her heart.

Bored and unhappy as the Lord of Hell, Lucifer Morningstar abandoned his throne and retired to Los Angeles, where he has teamed up with LAPD detective Chloe Decker to take down criminals. But the longer he's away from the underworld, the greater the threat that the worst of humanity could escape.

A unique team, consisting of a federal agent, a police officer and a conspiracy theory novelist, investigate the shocking reappearance of Alcatraz's most notorious prisoners, fifty years after they supposedly vanished.