The Buttercup Chain, based on Janice Elliot's novel, details the intense friendship among four people that turns tragic over the course of one summer. France (Hywel Bennett) and Margaret (Jane Asher) are first cousins, born of twin sisters on the same day. As children, they were brought up as brother and sister. The two possess such a strong platonic love for each other that they are incapable of loving anyone else. When Margaret travels to London to live with France, he picks out beautiful Swedish student Fred (Sven-Bertil Taube) to be Margaret's lover. For himself, he selects American girl Manny (Leigh Taylor-Young). As they spend their holidays traveling in Spain, the countryside of England, and Sweden, the foursome communicate with each other and try to discover the meaning of love.