The art broker, Alan Desland (played by Rupert Frazer), travels to Copenhagen to purchase a collection of paintings. While there, he meets a young secretary, Karin (played by Meg Tilly), who becomes his translator and assistant. Despite her reserved nature, Alan is taken by her beauty and charm. He decides to extend his stay and asks Karin to show him around the city. They bond over their mutual love of art and culture, and soon start a passionate love affair.
After a few weeks, Alan proposes marriage to Karin, which she accepts. However, Karin insists that they get married in a courthouse instead of a church, which raises some doubts in Alan's mind. Nevertheless, he agrees to her terms and they tie the knot. As they start their new life together, Alan begins to notice strange occurrences around him, such as mysterious footsteps and eerie music. He also encounters a mysterious old man who warns him about Karin's past.
Alan's suspicions are further aroused when Karin confesses to him that she was once involved with a cult that practiced black magic. She assures him that she has left her past behind and is now a changed person. However, Alan becomes increasingly obsessed with uncovering the truth about Karin's past and starts to investigate her past on his own. His growing paranoia and jealousy threaten to destroy their relationship and Karin's mental state.
The film deals with themes of love, obsession, and the supernatural. It is based on the novel of the same name by Richard Adams. The Girl in a Swing was directed by Gordon Hessler and was released in 1988.