Kitsune no Yomeiri tells a story about a fox demon (Rin) falling in love with a human. She used her magic to be with him, presumably in the man's dreams. As she wanted to be with him, she was willing to change herself, even her nature. But one day, she saw him with a woman, possibly his lover. Heartbroken, she killed him because it was the only way she saw to be able to keep him to herself.
The title refers to the sunshower phenomenon in Japanese folklore; they believed that sunshowers occur every time two fox demons (or a fox demon) get married, which is believed as a good omen. In Japanese folklore fox demons were mischievous tricksters and seducers/seductresses that trick humans by using their magic, and the Sanzu River is equivalent of the river Styx.