Childhood friends Koutarou Fujiki, Kajiwara, and Fuyuko thought they would always be together. They spent their youth in a strange love triangle, the two boys fighting over who would date Fuyuko. But suddenly one day, that life ends. Kajiwara goes missing, and Koutarou takes his sweetheart's hand in marriage. Now middle-aged, Koutarou is left to wonder—would his wife have chosen differently if Kajiwara were still around? On a visit to his hometown, the memories come rushing back...

Seshuusei Triangle is based on a web manga published on the author's Twitter and pixiv accounts from September 9, 2018 to October 6, 2018.

The series was published in English as Hereditary Triangle by Yen Press as a 2-in-1 omnibus on December 17, 2024.