K, a young schoolteacher in Tokyo, is quietly in love with Sumire, an aspiring writer who lives on the margins of ordinary life. Sumire, however, becomes captivated by Miu, an older, sophisticated woman who offers her both emotional and professional guidance. Drawn into Miu’s orbit, Sumire leaves Japan to travel with her, setting aside her writing as her world begins to shift in unexpected ways.

Then, without warning, Sumire disappears on a small Greek island. Miu contacts K, asking him to come and help uncover what happened. As K retraces Sumire’s final days, he is confronted with fragments of a story that resist explanation—suggesting that her disappearance may be tied to something beyond ordinary reality.

Blending quiet longing with surreal mystery, Sputnik Sweetheart explores unrequited love, identity, and the boundaries between worlds. It is a subtle and haunting meditation on connection and absence, where the search for another person becomes a search for meaning in a reality that feels increasingly uncertain.