Shukracharya is the guru of the demons and Vrihaspati, of the gods. But Shukracharya has the secret of Sanjeevani- ambrosia, which gives eternal life. So Kaccha is sent to learn to learn that secret.
Shukracharya treats his colleague's son like his own. Kacha promises to serve his guru for a thousand years. Devyani is Shukracharya's beautiful maiden daughter. The young couple's eyes meet and are stirred with life.
500 years into his service, the demons come to know what Kacha's real purpose is. They kill him and feed him to the wolves. Devyani begs her father for his life, Shukracharya cannot refuse. The demons kill him again and once again, on Devyani's please, Shukracharya revives him.
The demons then kill and cremate him and serve him to Shukracharya in wine. Shukracharya gets ready to revive Kacha again, when Devyani comes to him..When they realise where he is, Shukracharya teaches him the knowledge of Sanjeevani.. Kacha comes out and revives his guru who is akin to his father now.
And so when he gets ready to leave the ashram, he breaks Devyani's heart, he says they are brother and sister now, he cannot marry her. Devyani curses him and says that he shall never be able to use his knowledge- Kacha says he will give it to others instead.
He says she has cursed him despite knowing there were rules of conduct governing him.. for that error, no Brahman would ever marry her.
He says he did not wish to hurt her, he feels terrible that inadvertently he has hurt her so deeply. Devyani is looking away from him, her large eyes are full, she is holding onto a vine in the forest. Kacha's eyes are quiet and his index finger is lifted up as he spells out the rules that now govern their lives.
Rabindranath Tagore's Kacha Devyani