"K: Stray Dog Story" is a manga series that serves as a prequel to the "K" anime and light novel series, focusing on the backstory of Mikoto Suoh, the Red King, and his clan, Homra. The narrative delves into the origins of Homra, exploring how Mikoto, a seemingly aimless and detached individual, becomes the leader of a powerful group of Strains—individuals with supernatural abilities. The story is set in a world where seven Kings, each representing a different color and wielding immense power, govern various factions. Mikoto's journey begins when he encounters Izumo Kusanagi, a bartender who becomes his closest confidant, and Tatara Totsuka, a cheerful and loyal friend who later becomes a key member of Homra.

The plot unfolds as Mikoto, initially uninterested in leadership, is drawn into conflicts with other factions and the mysterious Dresden Slates, the source of the Kings' powers. His interactions with other characters, such as the ambitious and cunning Reisi Munakata of the Blue Clan, Scepter 4, highlight the tensions between the clans. The manga also explores the dynamics within Homra, showcasing the bonds of loyalty and camaraderie that form among its members, including Anna Kushina, a young girl with precognitive abilities who becomes deeply connected to Mikoto.

As the story progresses, Mikoto's reluctance to embrace his role as the Red King is tested by external threats and internal struggles. The manga provides insight into his character, revealing a man burdened by the weight of his power and the responsibilities it entails. The narrative builds toward the events depicted in the main "K" series, offering a deeper understanding of the relationships and conflicts that shape the world of the Kings. Through its exploration of Mikoto's past, "K: Stray Dog Story" enriches the overarching lore of the "K" universe, providing context for the motivations and actions of its central characters.