Plot summary from Wikipedia
The story follows the experiences of a young mongoose named Rikki-Tikki-Tavi (for his chattering vocalizations) after he is adopted into a British family residing in India, as a pet and as protection against venomous cobra snakes. After becoming friendly with some of the other creatures inhabiting the garden, Rikki is warned of two cobras Nag and Nagaina, who are angered by the family's presence on their territory. Accordingly Nag enters the house's bathroom before dawn but is attacked by Rikki. The struggle that ensues awakens the human family and the father kills Nag with a shotgun.
The grieving Nagaina attempts revenge against Rikki's humans, cornering them as they take breakfast on an outdoor veranda. While Nagaina is distracted by a female tailor bird Rikki destroys the cobra's unhatched brood of eggs except for one. He carries it to where Nagaina is threatening to bite the child Teddy while his parents watch helplessly. Nagaina, enraged, recovers her egg, but is pursued by Rikki-Tikki away from the house to the cobra's underground nest where an unseen final battle takes place. Rikki emerges triumphant from the hole declaring Nagaina dead. His subsequent role is to protect the family by keeping the garden free from any future intrusion by snakes.
Rikki Tikki Tavi