In 1970s London, a teenage outsider named Enn falls in love with a rebellious alien girl named Zan, who has come to Earth for a party. Together, they navigate the complexities of intergalactic culture and the trials of first love.

Margaret (Lena Headey) is a shy, pale, middle-class Englishwoman who is reluctantly engaged to her older, twittish neighbor Syl Monro (David Threlfall). Both bride- and groom-to-be still live with their mothers in the humdrum suburb of Croydon. However Margaret has been acting strangely ever since a vacation in Egypt, where she stayed with her mother's friend Marie-Claire (Catherine Schell). She secretly despises Syl, but does not resist when her mother Monica (Julie Walters), who has repressed the failure of her own matrimony, insists on marriage for the sake of social convention.