Taro Sakamoto, the legendary retired assassin now running a neighborhood convenience store, attempts to enjoy a rare family vacation at a hot spring resort with his wife and daughter. Old habits and hitman history threaten to land him in hot water when complications arise, including a misunderstanding that prevents his psychic employee Shin from accessing the baths at all. Alongside these present-day troubles, the light novel flashes back to Sakamoto's school days at the assassin training academy, following the young prodigy on a midnight heist to steal exam questions alongside his friends. Their mission grows complicated when they encounter the stern Professor Sato.

The collection extends to other members of the cast as well. A story focusing on the Segawa siblings places Mafuyu in a nightmare scenario when the pair attempts to work at a crepe food truck, a job that becomes dangerous when a suspicious customer arrives. The novel also revisits characters from the first light novel adaptation, Sakamoto Days: Assassin's Method, including a culinary tour featuring Gamagoori and Obotsu, where the former simply wants a quiet meal but the latter transforms every outing into an unexpected gastronomic adventure. A further installment set in an alternate reality imagines the characters as employees of Sakamoto Corporation, where Shin works as an executive and Akira from accounting alerts him to a possible internal corruption scheme.

Written by Renka Misaki, who also penned the first Sakamoto Days novel, Assassin's Blues presents seven exclusive episodes that expand the world of Yuuto Suzuki's manga series. The stories balance action and comedy while showing characters in situations rarely depicted in the main continuity, from part-time jobs to alternate universe scenarios, all illustrated with artwork by Suzuki himself. The light novel serves as a companion piece that deepens character relationships and offers fans a closer look at life both during and after Sakamoto's years as an unparalleled underworld legend.