Rodrigo de Arista is an aging aristocrat who has been hidden away for many years in a monastery by his widowed daughter-in-law Lucretia. Now he is older, penniless and half-blind -- but when he finds a letter hidden in his deceased son's belongings alluding to an adulturous affair conducted by Lucretia, he becomes determined to uncover the dark secret that resulted in the birth of one of his two granddaughters. But which of them is his true heir? To find out, Rodrigo will need the help of an unhappy and despised school teacher who tutors the granddaughters and in exchange, needs help carrying out a desperate plan of his own.