The four islands at the southern end of the world, Karoku, Kada, Uka, and Shamon, are said to belong to the Sea God, having formed from the husk of the Serpent God. Each island is ruled by its own lord, but the lords are chosen not by bloodline but by the oracle of the shamaness queen who serves the Sea God. The Daughters of the Sea God gather under this queen and are married to the lords according to the divine oracle. By taking these daughters as brides, the islands gain the blessing of the Sea God and prosper.

By the Sea God's decree, a young woman named Ran is destined to marry Kei, the young lord of Karoku. However, her father was executed by Eikun, the previous lord of Karoku. Filled with unease at her fate of marrying the son of the man who killed her own family, Ran sets out toward her wedding ceremony in a small boat under the moonlight. The central narrative arc follows this marriage tale, depicting the happiness of the daughters chosen by the gods while exploring the dramatic tension between duty and personal tragedy as Ran navigates her relationship with Kei amidst the dark history between their families.