London. During broad daylight, Mary Hilton shoots rich society woman Lucy Carpenter, outside her apartment. The murder is passionate; Hilton empties the gun and, as a crowd gathers, stands defiant, ready to face the consequences of her actions.
Prison, several months later and post-murder trial. It is the start of Spring. Mary has been sentenced to the death penalty but is hoping for a reprieve. Six prison officers work in rotation to guard her. Each has a different opinion of Mary and her treatment varies from liberal understanding to strict control. One guard, MacFarlane, is particularly affectionate towards Mary.
Prison life is full of mundane routine. Mary is isolated from all other inmates and constantly subject to visits from the Chaplain and Doctor. She has to follow a strict regime. One of her few pleasures is the daily exercise taken in the yard, where she is free to daydream. She begins to recall the events leading to her imprisonment...
While working at a perfume counter in a department store, Mary encounters a charming customer, Jim Lancaster. Although she's already married, and despite Jim's dubious reputation with women, Mary falls in love with him and they begin a passionate affair. Sure of her love, Mary leaves her husband, Fred, but realises too late that Jim doesn't reciprocate her love. She learns that he has started seeing another woman, Lucy Carpenter, and after learning of his deepening love for Lucy, Mary leaves him.
The following New Year's Eve, Jim turns up at Mary's flat unexpectedly, carrying a gun and threatening suicide. At first, she manages to calm him, hiding the gun, but he runs away to find Lucy again. After a few hours without hearing from him, she rings his flat, only to find he has killed himself. Disgusted by both his death and Lucy's dismissive reaction to it, Mary begins to hate, and finally uses Jim's gun to wreak her revenge.
Prison life remains intolerable. The routine is only broken by visits from Mary's mother and brother and, occasionally, Fred. None of these are any help to Mary. An experienced visitor of condemned women, Miss Bligh, provides some comfort, but only serves to strengthen her belief that her homicidal actions were justified. Her greatest comfort comes from MacFarlane, a woman who shares Mary's experience of difficult families and who also lost a man to another woman. She doesn't judge Mary's emotions, understanding her rage while disapproving her actions.
The day arrives when Mary learns of her fate. The governess informs her that the home secretary has not seen fit to recommend a reprieve.
Mary's last days are experienced in a state of delirium. Her routine transforms into a series of final actions: her last walk, last sleep, last meal. She understands the strange knowledge shared by all those around her: they're going to kill, as she did once, only this time its legal. On her final meeting with her mother, she hides her tears. With Miss Bligh, she is given advice about coping with the last night. The burden of her sin is now passed to her executioners, but Mary feels she is not ready to die and dreads the morning.
MacFarlane cares for her during the night, sitting by her bed and offering words of solace. When she awakes, MacFarlane is gone, and Mary is prepared by two prison officers. Unable to eat, she takes a few drags of her last cigarette, but is interrupted by the Chaplain. Together, they leave for the execution chamber. As the chimes of the chapel mourn her passing, Mary's cigarette is left burning.
