El Juramento tells the story of Santiago, a dashing hero who sets out to avenge the death of his brother, Diego. He discovers that Diego killed himself after the beautiful Alma, a depraved gold-digger, aborted a child that she falsely claimed was Diego's and refused his marriage proposal via a "Dear John" letter. Thanks to the village inhabitants - who at first are hostile but become friendly - Santiago discovers pieces of the puzzle that led to the tragedy. Yet all he knows of the mysterious woman's identity is a necklace, a gift from Alma and her cousin Andrea's late grandfather. The two cousins own identical necklaces each of which has a gold letter "A."

Santiago seeks revenge on the faithless lover. The clues bring him to a hacienda in Mexico, where Diego was once a trusted worker. A wealthy man, Teodoro Robles Conde, owns the house and lives with his two nieces—and our hero knows one of them caused Diego's suicide, but which one?

Santiago meets the pair of rich, gorgeous cousins, treacherous Alma and virtuous Andrea. The first time he sees Alma, she is wearing Andrea's necklace, because Alma left her own necklace in Diego's possession. At first, Santiago acknowledges that either of the cousins might be the guilty one. Partly as a result of a series of mishaps and false rumors, mostly contrived and spread by Alma, but also because Santiago is attracted to Andrea and assumes his brother would have preferred Andrea, too, Santiago decides that Andrea is the one who betrayed Diego. He decides to make her life miserable as revenge for his brother's death. As a result, Andrea endangers herself for her cousin's wrongdoings.

Santiago unleashes his vengeful plan: He flirts with Andrea, seduces her and makes her fall in love to the extreme of making her his bride. After the wedding, he drags her to the small remote village where Diego killed himself. Santiago does not see that both he and Andrea are really victims of Alma, whose angelic face hides a demonic soul. Meanwhile, Santiago finds himself falling in love with Andrea but fights hard to resist his feelings in order to keep his oath to Diego.

When Santiago finds out the truth, it seems that all is lost. Andrea abandons him because he did not trust her. Alma goes unpunished for her deeds while her Aunt Luisa covers her trail. The journey of deception leads to intrigue, passion, confusion, revelations and betrayal. As with all telenovelas love wins in the end—whose love and at what cost? Is it possible to love that which we most hate?