In the summer of 1942, Marc Roussillon, a heroic French pilot flying with the RAF, draws his wife Simone into a plot to save his best friend Jean-Claude, who has been captured by the Germans. Aware of her influence upon Gerhard Hauptmann, a German officer, Marc urges Simone to persuade Gerhard to help her set Jean-Claude free. But, unknown to Marc, Gerhard agrees to help her only if she will sleep with him. Just before Jean-Claude is freed, Simone joins Gerhard for ten days in Switzerland. Falling in love with Gerhard confuses Simone and makes her feel guilty. She still loves Marc, but she also needs Gerhard. Their new, complicated relationship alters both Marc's and Simone's perception of themselves and of each other. Yet, in spite of their emerging self-knowledge, their obsessions drive them into darker territories.

When he discovers that Simone has fallen in love with Gerhard, Marc tells Simone that they can save their marriage if she joins him in his plan to kill Gerhard. Lying, he convinces her that Gerhard is responsible for the deaths of many innocent people. Now Simone consents to Marc's plan. She will meet Gerhard at a Swiss ski resort. There, Marc plans to kill Gerhard.

But Marc's plan unfolds in ways that he does not anticipate. Simone discovers that Marc lied about Gerhard. Now she hurries to save Gerhard from Marc's revenge. In violent episodes that occur in Switzerland and in England, Marc, Simone, and Gerhard play out the turbulent resolution of their triangle.