Because of her vivid imagination and unorthodox behavior, Catherine (Danièle Delorme), a young Irish colleen, is considered to be a lunatic. In one of her delusional moments, she imagines that her dead brother is calling for her. She searches for her brother within the ranks of the Irish rebel organization responsible for his death, falling in love with the man (Henri Vidal) who, unbeknownst to her, ordered the killing. Upon learning the truth, Catherine lives only for revenge, leading to an explosive, almost operatic climax.