Lal Patthar is a 1971 Indian Hindi-language action drama film, produced by F. C. Mehra, and directed by Sushil Majumdar. The film is a remake of Majumder's own Bengali film Lal Pathore (1964). The film stars Raaj Kumar, Hema Malini, Raakhee, Vinod Mehra, Ajit. The film is one of the rare films where lead actress Hema Malini played a negative role, being a jealous mistress of a zamindar who tries to frame his young wife as an adultress. Her performance received accolades. The music of the film was composed by Shankar–Jaikishan.

The film was shot on some minor outdoor locations and at Mehboob Studios, as well as at Natraj Studios Bombay.

A family goes to Fatehpur Sikri for a picnic. There they encounter an old man, who says that every stone of the fort is steeped in a story written with blood. The old man asks the family to see Lal Patthar, the red stone, which the family retorts is not on the guidebook. The old man says that the guidebooks don't reveal the history of the fort. The old man is paranoid and sees blood even in a glass of plain water. The old man narrates the story of Rai Nagar. The first ruler of Rai Nagar Raja Raghav Shankar Rai (Ajit) was a bandit. Raghav sexually assaulted a peasant woman named Sonmai during one of his village raids, and made her his wife. Sonmai cursed the entire family with insanity.

Now in the 7th generation, Gyan Shankar is the only male heir of the house. His grandfather is Raja Ram Shankar Rai (D. K. Sapru) and is mentally unstable. As a kid, Gyan stopped his drunk father Anand Shankar from sexually assaulting a maid in their palace. Gyan Shankar was sent to his uncle in Allahabad, where he studied history and psychology. Gyan returned home with Chhotu (Paintal), his servant. To become the ruler, Gyan has to marry, but he has decided not to, as he fears that he will ruin a woman's life due to Sonmai's curse.

Raja Gyan Shankar Rai (Raaj Kumar) is also known as Kumar Bahadur. He goes to the jungle to hunt a man-eating tiger. After killing the tiger, Gyan manages to fight off a group of bandits that were carrying a palanquin. The bandits escape and Gyan finds a woman named Saudamani (Hema Malini) inside the Palanquin.

Gyan brings Saudamani home and is smitten with her beauty. Gyan finds that Saudamani is the widow of Gokul from a nearby village, which was plundered by bandits a few days ago. Gyan sends Saudamani back, but Gokul's mother (Leela Mishra) declares her unchaste and refuses to let her back into the house. Gokul's mother is scared by the guns of Gyan's guards and is forced to accept Saudamani. Back at the palace, Gyan starts to drink. Gyan finds that Saudamani is being tortured in her own home and brings her back to his palace.

Calling her Madhuri, he invites the lower-class woman to his rich home and eventually has sex with her. But Gyan refuses to marry Madhuri. Still, Madhuri has full control over the house and has Gyan wrapped around her fingers. Gyan also educates Madhuri. Initially, Gyan is enamored with Madhuri, but with time he realizes that Madhuri cannot be trained (in singing) or educated. She has a middle class thinking, limited ambition and a negative mindset. He becomes disgusted with Madhuri. Soon, ten years have passed and Gyan is no longer a young man.

Gyan runs away from his palace, and comes across a singing function, where he meets a much younger woman, Sumita (Raakhee). Gyan finds that Sumita's father Harishchandra Chakraborty is a gambler and in heavy debts. Gyan organizes a financial deal with her father and marries her. Sumita's mother Madhu (Dulari) was vehemently against the marriage but was overruled by Harish.