Kokou no Danna-sama wa Mitsu o Tashinamu ~Taishou Yami Barai Kitan~

In the Taisho era, Iroha, a commoner orphaned after her mother's death, finds herself alone in the world. Suddenly, her half-sister, Chigusa of the Kazanin family, arrives and asks her to take her place as the bride of the handsome Baron, Rei Toitsu. Though bewildered by this sudden marriage, Iroha approaches the ceremony with a longing for family in her heart. Yet, immediately after the wedding ends, Rei snatches Iroha up, carries her away, and pins her down. The Itotsu family bears a mission to purge darkness, and for that purpose, they claim they must consummate the union with Iroha—but...?