Beta is the story of Raju (Anil Kapoor), the only child of a widowed multi-millionaire father who can provide his son with anything he wants except Raju's only desire--a mother's love. Raju's father believes he can please his son by marrying Laxmi (Aruna Irani), thinking that she will care for Raju as her natural son. Raju becomes devoted to his stepmother, does whatever she asks of him; she convinces her husband that Raju should remain naïve and uneducated otherwise he would work for others rather than be self-employed. Raju grows older his step-mother increasingly isolates his father from the family, is considered mentally incompetent and eventually locked within a room of the family home.
Raju meets Saraswati (Madhuri Dixit) and following she being abducted and assaulted at a fair, he recues her. The two fall in love and despite the villagers believing she is no longer chaste he marries her. Saraswati discovers that her step-mother-in-law's motherly love for Raju is a rouse in an attempt to discredit the sanity of Raju's father and thus unable to interfere with the plan Laxmi has for the family fortune to be diverted to Raju's step brother, the natural result of the marriage between his father and step-mother. He also shares in the greed for the family fortune. A battle of wills between the step-mother and the step-daughter-in-law ensues.
Laxmi sees that her influence over the family is being challenged by Saraswati who insists that Raju's father leave behind his prison and return to the family circle as there is nothing wrong with him. And she makes Raju aware of the intentions of his step-mother. Laxmi starts to abuse and embarrass Saraswati with all the family members present. Saraswati is ready to leave but then, to protect her husband and her house from Laxmi's intentions, she decides to apologise to her mother-in-law. Saraswati then cleverly starts exposing Laxmi's every effort and intention in a dignified manner so that her husband will not be offended. The medical school degree of Raju's step-brother was bought rather than earned through studies and Laxmi is set up to slip on the pavement so that the over-protectiveness that she has instilled in Raju will force her remain in bed as he will serve her without pause; Laxmi never has a moment to herself and her scheming.
Saraswati becoming pregnant prompts Laxmi to attempt to kill Ragu's wife and unborn child with poison, saffron and milk. Saraswati discovers this and returns to her direct approach by telling Raju. He continues to refuse to believe the treachery although she takes an oath upon her unborn child's life. Raju defends his step-mother and offers to prove that Saraswati is wrong. Raju drinks the milk and then coughs up blood. He comes to the realization of what Saraswati said was all along his step-mother's intentions. In his usual innocent manner, he asks her why, that all she had to do as his mother was simply ask for the wealth--he would have happily agreed to give her it all. He tells Laxmi that his wish to die in peace would be accomplished if she, to at least once with a clean heart, call him 'her son'. His words so deeply touch Laxmi that she realises her cruelty has been directed at the only son who all along had ever loved her. Raju's step brother physically confronts his mother as he wants to continue with the plan but the dying Raju saves her.
The film concludes with Raju recovering, agreeing to give up to his mother his worldly possessions, and leaving home with his wife and father. At the last moment, Laxmi begs him not to leave, claiming to have learned the error of her ways. She tears-up the legal papers and tells him that all she wants is nothing more than 'her son'.