Black story (La historia negra de Peter P. Peter) (1971)

Pedro is a successful novelist who writes crime novels under the pseudonym Peter P. Peter. After being widowed, he marries Beatriz, who is also a recent widow. His new married life turns out to be hell, since his wife makes life impossible for him and he has fallen madly in love with his secretary Carlota. The experience that his novels have given him leads him to hatch a plan to poison his new wife, but she also plans to kill him in order to marry Jerónimo.