In the Ramayana, when Lord Rama, Mother Sita, and Lakshmana were living in a place called Panchavati during their exile, Ravana's sister, Surpanakha, arrived there. Surpanakha was a demoness, and when she saw Lord Rama, she became infatuated with him.
Surpanakha proposed marriage to Lord Rama. Lord Rama politely declined, saying that he was already married and deeply loved his wife, Sita. He suggested that she approach Lakshmana instead. Lakshmana also rejected Surpanakha's proposal.
This made Surpanakha very angry, and she attempted to attack Sita. To protect his wife, Lord Rama ordered Lakshmana to teach Surpanakha a lesson. Lakshmana immediately cut off Surpanakha's nose with his sword.
Crying and wailing, Surpanakha went to her brother Ravana and narrated her plight. She incited Ravana to take revenge on Lord Rama and abduct Sita. This incident gave the great epic of Ramayana a new turn and eventually led to the war between Lord Rama and Ravana.
The story of Surpanakha's nose being cut off teaches us that evil and injustice are bound to meet their end. Lord Rama and Lakshmana, by following the path of righteousness and truth, protected Sita and punished Surpanakha for her improper conduct.