Jack - known as Jacko - Palmer is the dedicated area secretary of a trade union. He arrives home on the hottest summer's night for 11 years to find a colleague, Frank Stephens, waiting for him. Stephens warns that there is trouble brewing regarding the proposed promotion of a Jamaican worker, Gabrielle Gomez, to charge-hand. Jacko is outraged to hear that the promotion is opposed on racial grounds and vows to help Gomez. Stephens leaves and Gomez's white wife, Judy, calls by. Again, Jacko pledges his support.

Jacko receives a call from Ken Mitchell, one of the objectors to Gomez. Jacko shouts down the telephone until he hangs up. Meanwhile, the Palmers' daughter, Kathie, is walking by London's docks with her young Jamaican boyfriend, Sonny Lincoln. Racist graffiti and the reaction of the dockers reminds them of their differences. She asks him to call at her house that night to meet her parents.

As Kathie makes her way home, her mother, Nell takes a call from Mitchell which upsets her. Nell later tells Kathie that Mitchell claimed she had been seen at the swimming baths with a black man. Kathie confirms that it was true. She has known him for six or seven months, having met him at her evening class. She reveals she wants to marry him. Nell is distraught and says she does not want him visiting. She says Kathie will be ruining her life and is disgusted by the idea of her having black children. Upset, Kathie runs upstairs.

Jacko takes a call from Bill Harrison asking him to go to Manchester that night to represent the union on television. Nell, needing him home, tells him not go. She tells Jack that Kathie has fallen in love with a West Indian. Jacko doesn't think there is anything they can do and Nell accuses him of always having cared more about his union than his family.

Jacko talks to Kathie about her attachment to Sonny. Her grandfather tells her how class differences had kept him and the woman he loved as a young man apart and assures her that the hurt fades. Kathie accuses them all of caring only about colour. Nell runs off distraught. At that moment, Sonny calls at the door. Jacko invites him to talk in the yard. Kathie goes to talk to her mother.

Jacko explains that for the sake of Kathie's happiness, Sonny should leave and never return to her. Jacko shows him the door. Sonny accuses Jacko of being a hypocrite - opposing racism at work but espousing it in his home life. Just as Sonny is about to leave, Kathie returns and persuades him not to walk out.

As Kathie and Sonny go indoors, Judy calls. Nell asks Judy to tell them about the miseries of choosing a mixed relationship. Nell pleads with her to discourage Kathie from marrying a black man but, despite the privations she has experienced, she will not. Judy leaves. Kathie, having decided to leave the family home that night, goes to packs her things. She is given money by her grandfather.

Outside, Jacko and Nell talk. Nell reveals how she has felt neglected by her husband as he devoted himself to his union. She has always wanted a nicer home, but for the sake of appearances Jack maintained that he must live in the same standard of housing as the workers he represents. She suggests it was Kathie holding them together. She says she only wanted Kathie to be happy and becomes distraught again at the thought of Kathie becoming caught in a marriage that will cause her misery.

Kathie returns and says goodbye to her parents. As she turns to leave with Sonny, Nell grabs a knife and runs to attack them, but is disarmed by Jacko. At Sonny's suggestion, Kathie agrees to remain there for the night, and will rejoin her fiancé the next day. Sonny leaves.

Knowing what poor accommodation will be available to them as a mixed couple, and having heard Nell's thoughts on their home, Jacko offers Kathie the house for when she marries. Jacko promises Nell that they will make a fresh start and that he will try to change his ways.