Alice Merren is in prison awaiting trial for the murder of her former Bletchley Park colleague. Jean, believing Alice is covering for someone, begins to reunite the circle to help establish the innocence of one of their own. The investigation leads to a young woman who seems to have had a relationship with the deceased and in whose home they find documents suggesting a military cover-up of a chemical spill. The women use the skills honed at Bletchley Park - and take dangerous chances - to try to exonerate Alice. Since Alice refuses to participate in her own defense, the women begin to investigate the murder. Clues found in the murdered man's house lead to Lizzie, a woman with whom he had a relationship. In Lizzie's apartment, the women find a classified military file and press clippings relating to a chemical spill. Eventually they track down Lizzie, who fears her life is in danger. Susan deduces why Alice is willing to face the death penalty.