The title "Sputnik Sweetheart" is derived from a miscommunication regarding the Russian satellite "Sputnik," which becomes a metaphor for the characters' emotional distance and disconnection from one another. The novel explores themes of love, longing, and the existential search for identity and purpose. It delves deeply into the nature of unrequited love, as K secretly harbors feelings for Sumire, while Sumire becomes obsessed with Miu, who remains emotionally distant due to a traumatic experience from her past that left her "split in two."

Murakami weaves a narrative that blends the mundane with the surreal, employing his signature style of magic realism. The story takes a mysterious turn when Sumire suddenly disappears while on the Greek island with Miu. As K arrives to help search for her, the boundaries between reality and imagination blur, and he is forced to confront his own feelings and existential uncertainties.

"Sputnik Sweetheart" is characterized by its introspective tone, exploring the inner lives of its characters and their struggles with isolation and emotional connection. Murakami uses sparse, evocative language to create a sense of melancholy and ambiguity, leaving readers to ponder the unresolved questions and the nature of the characters' desires and realities.

The novel reflects Murakami's exploration of existential themes, drawing on influences from Western literature, philosophy, and pop culture. It raises profound questions about the nature of human relationships, the quest for personal meaning, and the mysteries of the subconscious mind. Through its blend of everyday life and the fantastical, "Sputnik Sweetheart" offers a haunting and thought-provoking journey into the depths of love, identity, and the search for one's place in an ever-expanding universe.

The novel has been praised for its lyrical prose, enigmatic narrative, and the emotional depth with which Murakami portrays the complexities of human emotion. It is a compelling read for those interested in modern Japanese fiction, as well as for readers who appreciate stories that challenge conventional narrative structures and delve into the psychological and metaphysical realms.