Diligent 25-year-old novelist Yukari Nagaya is completely content living alone in the spacious apartment she purchased after her older twin sister, Ayaka, got married. The pair of siblings have grown apart over the years since their parents' divorce, and as such, Yukari is left taken aback when Ayaka visits her home one day with an odd request—to let a friend temporarily stay over.

This friend, Naomi, is an avid traveler who became friends with Ayaka when he helped her catch a purse-snatching thief during her honeymoon in Hong Kong. Suspicious of their relationship, Yukari reluctantly agrees to let him live in her apartment until he finds a job with a stable income. Thus unfolds Futago, a story of love, marriage, family, and sisterhood.

Included one-shots: Volume 5: Papa Mama Futago, Eri-chan no Kare