English teacher riko and school nurse kyousuke are a couple who've been living together for about a year. One day, kyousuke's cousin comes back to japan after studying overseas, and moves in with them. While the situation makes riko uncomfortable, she can't bring herself to complain when she sees how much kyousuke adores him... On the day of his cousins arrival, kyousuke gets caught up at work and it falls to riko to meet him. To her astonishment.. Its mizuki, a student she tutored in her...! Sheken, riko pretends she's forgotten about the incident... But mizuki has other plans in mind...!