Tidelands follows the story of a former criminal who returns home to the small fishing village of Orphelin Bay. When the body of a local fisherman washes ashore, she must uncover the town's secrets while investigating its strange inhabitants, who are a group of dangerous half-sirens, half-humans called "Tidelanders".

In 1817, Tessa Alvarado lives by day in Southern California as a young noblewoman of wealth. At night she is the Queen of Swords, a masked protector who uses whip and sword to punish evil.