Cleavage 1

Yuuto and Erika begin to live alone after their father's job transfer to Sapporo. They are siblings, but not blood related, as Yuuto's widowed father married Erika's mother. Finding themselves alone, Yuuto and Erika initiate a sexual relationship, but Yuuto's art teacher, Sayaka Ichinose, notices the change and takes an interest in what is going on.