There are four islands at the southern end of the world: Karoku, Kada, Uka, and Shamon.
The lords of each island are to marry the "Daughters of the Sea God," who are chosen by the oracle of the Sea God. By marrying these daughters, the islands will gain the blessings of the sea god and prosper.

This is a marriage tale depicting the “happiness” of the daughters chosen by the gods.

(Source: Kodansha, translated)