Once upon a time, a king named Ashwapati ruled in the Madra country. He was extremely kind and religious by nature, but he had no children. On the advice of astrologers, he performed a yajna to obtain a child.
After this, a beautiful daughter was born to him, who was named Savitri. When Savitri grew up, she chose a young man named Satyavan for marriage.
When Devarshi Narad came to know about this, he told Savitri's father Ashwapati that the Satyavan whom Savitri was going to marry had a very short lifespan, so she should choose some other groom.
As soon as he came to know about this from Narad Muni, King Ashwapati tried hard to persuade Savitri, but Savitri remained adamant on her point, arguing that Hindu women choose a man as their groom only once. After this, King Ashwapati married Savitri to Satyavan.
Satyavan's father's name was Dyumatsen, whose kingdom had been snatched away. Both Satyavan's mother and father had also lost their eyesight and they used to live in the forest.
After marriage, Savitri also started living in the same forest with her husband Satyavan and serving her parents-in-law along with her husband. After a few days, Narad Muni came to Savitri and told her that the time for Satyavan's death had now arrived.
When Savitri came to know about this, she started fasting to obtain a long life for her husband. It is believed that Savitri kept her fast even on the date that Narad Muni had told for Satyavan's death.
On that day, when Satyavan started going to the forest to cut wood, Savitri also went with him. While Satyavan was cutting wood in the forest, suddenly he started having severe pain in his head and he lay down with his head in Savitri's lap.
Savitri suddenly saw that Lord Yama had arrived there with his messengers. After this, when Yamraj started going towards the south direction after taking away Satyavan's life, Savitri also started following behind him.
Seeing Savitri following, Yamraj said, "O devoted wife! You accompanied your husband as far as you could, now you go back." Then Savitri said, "Wherever my husband stays, I also have to stay there with him. This is my duty towards my husband."
Hearing Savitri's words, Yamraj became pleased and asked her to ask for a boon. Then Savitri asked him to restore the eyesight of her parents-in-law. Yamraj said "Tathastu" (so be it) and asked Savitri to return, but still Savitri kept walking behind him.
Seeing Savitri's devotion to her husband, Yamraj once again asked her to ask for a boon. Then Savitri asked him for the lost kingdom of her parents-in-law. Yamraj said "Tathastu" and fulfilled this wish of hers too, but Savitri still kept walking behind Lord Yama.
Seeing Savitri fulfilling her duty of a devoted wife, Yamraj once again asked Savitri to ask for a boon, then she said, "I want to become the mother of 100 sons of Satyavan." Seeing Savitri's devotion to her husband, Yamraj granted this boon as well and returned Satyavan's life.
In this way, Savitri brought back not only her husband's life but also the eyesight and kingdom of her parents-in-law.
It is believed that by reciting or listening to this holy story of Vat Savitri, married women receive the boon of lifelong happy marriage.