Over the Threshold is about intermarriage between Japanese and Westerners. The filmmakers are a husband and wife team who met during their studies at Britain's National Film & TV School in London. The film documents their first visit to Japan together, during which husband Tezuka introduces his non-Japanese wife to his family (his father coincidentally was the creator of the cartoon series Astro Boy).

The point-of-view is largely that of the 'outsider', and much of the film is devoted to her attempts to find out what it means to 'fit in' in Japan. At a deeper level, the film amounts to a socio-cultural study of a Japanese family, who seem to adapt surprisingly well to having a 'geijan' or foreigner in their midst. This aspect is strengthened by the incorporation of interviews with other Western women who have married Japanese husbands.