Eternal Duke & Alma's Sin

An ending book is a proof of one's life that one leaves for a loved one. Alma, a girl who is not good at socializing, has taken over the shop that makes these books from the late Jiji. Recently, she has been dreaming about a man and woman she doesn't know every day, but one day, a customer turns out to be the person from her dream!? Rumor has it that he is immortal, so why is he creating an ending book? What is the relationship between him, who she thought was just a dream, and Alma, who actually exists...?