A young man -- starving, nearly catatonic and barely responsive -- is confined in a sanitarium. He is taken on a field trip along with other residents to the city's red-light district. There, he encounters by chance a woman from his past, triggering a series of flashbacks. We see that he was traumatized as a child, when he and his family were circus performers, and when he witnessed a murder/suicide: his father -- an American expatriate living in Mexico under suspicious circumstances -- cuts off the arms of his beloved mother, a possessive wife and religious fanatic who led a heretical church called "Santa Sangre (Holy Blood)," the members of which worshipped a martyred girl whose arms were severed by her father following her rape, and then commit suicide. Back in the present, buttressed by shock of his remembrance, he escapes the sanitarium and, in a series of hallucinations and dream-fulfillments, believes he has rejoined his armless mother. He "becomes her arms" and the "two" undertake a grisly campaign of murder and revenge.