In 1947, Vinayak Rao tells his 14-year-old son Pandurang about the Goddess of Prosperity who gave birth to 160 million gods. Hastar, her first and most beloved offspring, was greedy for all the gold and food. The other gods attacked Hastar, but the goddess saved him on the condition that he could never be worshipped and would be forgotten by history.
Vinayak's mother serves the local lord, Sarkar, in his mansion. This includes offering him sexual services. She hopes to acquire a single gold coin kept with his Hastar statue. Vinayak and his brother Sadashiv worry about having to feed the monstrous old woman who is their grandmother chained in a separate room. Sarkar later dies and the mother proposes leaving Tumbbad for Pune. Vinayak insists on finding the treasure that is rumoured to be hidden somewhere in the mansion. Sadashiv is badly hurt after falling from a tree, forcing his mother to take him for help. She tells Vinayak he will have to feed his grandmother. She also warns him that if the old woman wakes up, he should say the name "Hastar" to make her sleep. Sadashiv dies on the way to the doctor.
The mother directs her carriage driver to Sarkar's mansion where she retrieves the gold coin. The monstrous old woman attacks and shackles Vinayak who eventually remembers Hastar's name and uses it to make her fall asleep. Desperate to escape his life of poverty, Vinayak returns to Tumbbad. The old woman warns him he will be cursed and become a monster like her if he touches the treasure. She ultimately explains that the well leads to the goddess' womb where Hastar dwells, in exchange for ending her prolonged suffering. Vinayak keeps his promise and sets her on fire, thus killing her. Although Hastar stole the goddess's gold, he was unable to acquire her grain. Therefore, he desires flour. Vinayak physically trains to climb up and down a long rope and makes dolls out of dough to lure Hastar inside the goddess's womb. While he is distracted by the food, Vinayak swipes at Hastar's loincloth containing the gold. Vinayak regularly repeats the procedure to steal dropped coins. He offers his first gold coin to the opium merchant Raghav to pay off a debt. Each time he needs more money, Vinayak returns to Tumbbad to steal from Hastar.
Raghav openly wonders about the treasure rumoured to exist in the Tumbbad mansion while also questioning why Vinayak can only retrieve a few coins at a time. Sergeant Cooper pressures Raghav about the money to acquire his opium den permit under an arrangement they made previously. Vinayak and his wife give birth to their son Pandurang. When Sergeant Cooper gives him only two days to come through with the money, Raghav sells his widowed daughter-in-law to Vinayak as a mistress. She confesses to Vinayak that she has been paid by Raghav to make sure he stays in Pune for two days. Vinayak follows Raghav to the Tumbbad mansion and tricks him into descending into Hastar's well. Hastar viciously attacks Raghav and he is mutated into the walls of the womb. Vinayak descends after him and performs his usual ritual in creating the circle of grain and retrieves coins. Vinayak burns Raghav alive before leaving to end his suffering. Vinayak takes Pandurang to Tumbbad for stealing. Pandurang is told not to bring the dough doll down the well as it is just practice. Once in the womb, Pandurang reveals that he brought a dough doll with him. Hastar unexpectedly attacks. While his father is only concerned with escaping, Pandurang manages to steal several coins. Vinayak and Pandurang make it to safety, though Vinayak chides his son for nearly getting them killed. Pandurang suggests stealing Hastarâ€™s loincloth instead of going back everytime to only retrieve a small amount of coins. Vinayak and Pandurang make dozens of dough dolls to distract Hastar before returning to the well. However, the two of them become trapped inside the goddess's womb when separate Hastars appear for each one of the dolls they brought. With no hope of escaping, Vinayak ties the dolls to his body to act as bait. Vinayak climbs the rope. Dozens of Hastars burst into flames as they come into contact with the protection circles while pursuing him. Once the coast is clear, Pandurang climbs back to the surface. He finds his father mutated into a monstrous form outside the mansion. Vinayak presents the stolen loincloth, but Pandurang refuses to take it. Sobbing at seeing what his father turned into, Pandurang reluctantly sets Vinayak on fire.