The best place to start looking for the answer to this question is the third paragraph of the story. This paragraph tells readers that a high summer flood washes Rikki-tikki out of his burrow and carries him down a roadside ditch. Rikki-tikki clings on to anything he can find in an effort to prevent himself from drowning, and his efforts exhaust him to the point where he passes out. When Rikki-tikki finally regains consciousness, he is in the middle of a garden path in bright sun. This is when Teddy shouts out,
Here's a dead mongoose. Let's have a funeral.
Fortunately for Rikki-tikki, Teddy's mother thinks that the mongoose might not be dead. She suggests that they dry him off and give him a closer look. Teddy, his mother, and his father take Rikki-tikki into the house, wrap him in warm blankets, and set him by the fire. Rikki-tikki eventually wakes up and becomes the curious and fierce mongoose that decades of readers have come to know and love:
"There are more things to find out about in this house," he said to himself, "than all my family could find out in all their lives. I shall certainly stay and find out."