The novel Sputnik Sweetheart explores the complex relationship between two women, Sumire and Mio, and the narrator. It delves into themes of love, loss, identity, and the search for meaning, set against a journey through Europe and Sumire’s mysterious disappearance on a Greek island. The story features internal monologues and philosophical reflections, with lyrical elements on music and literature, as the narrator tries to understand the events and his emotions.