Quartet follows the lives of four musicians—Maki Maki, Suzume Sebuki, Yutaka Iemori, and Tsukasa Besshu—who unexpectedly meet and decide to form a string quartet. They move into a villa in the resort town of Karuizawa to practice and perform together during the winter months. However, the formation of the group is not as coincidental as it seems, as each member harbors deep secrets and hidden agendas that slowly unravel as they live together. As the series progresses, the quartet navigates the complexities of their personal histories, failed dreams, and the mysterious circumstances surrounding Maki's missing husband. The drama masterfully blends dark comedy with suspense and melancholic character studies, exploring themes of loneliness, the pursuit of artistic perfection, and the fragile nature of human relationships. It is a poignant look at four 'losers' in life who find an unlikely, albeit complicated, sense of belonging in one another.