During a certain spring break, Saki's son Yuuki is invited by Masaru's mother, Yurika, to go on a hot spring trip with two families. In fact, Yuuki had long admired Yurika's beauty and sex appeal. At the inn, thanks to the mixed-gender hot spring, he finally witnesses Yurika's naked body. Yuuki, whose excitement doesn't cool down, sneaks into Yurika's bed and has sex with her at night. And then Saki, the mother, also finds out about the two's affair, and Saki, who loves Yuuki very much, also becomes horny...
