The Asahina family, consisting of thirteen brothers and their new stepsister, takes a trip to a hot spring resort. The protagonist, the lone young woman who has recently become part of this sprawling all-male household, joins her thirteen stepbrothers for a relaxing getaway filled with温泉, ping-pong, shared meals, and other playful activities. With so many siblings gathered in one place, the peaceful vacation quickly turns into a series of comedic mishaps and unexpected romantic moments. Each brother, with his distinct personality and hidden feelings, vies for the heroine’s attention, creating a chaotic yet warmhearted atmosphere. The story adapts a popular long-form episode from the drama CD 13Bros.MTG, expanding it with original manga-exclusive scenes that were not present in the audio drama. The central conflict revolves around the heroine navigating her complicated relationships with her thirteen stepbrothers while trying to enjoy a simple family vacation, all set against the scenic backdrop of a traditional Japanese hot spring town.