Sputnik Sweetheart is a story of an unusual love triangle between K, Sumerie, and Miu. A school teacher, an aspiring novelist, and a businesswoman. All three of them seek the love of the other. As the friendship between K and Sumerie flourishes, Sumerie falls in love with Miu, 17 years elder woman. As K struggles between the decision to confess his love for Sumerie or to let go of his feeling, he receives a call troubled call from Miu, Sumerie has suddenly disappeared from a Greek Island...