A high summer flood is responsible for placing Rikki-tikki into the care and household of Teddy's family.
Paragraph three of the story tells readers that a summer flood washed Rikki-tikki out of the burrow that he lived in with his family. Rikki-tikki almost dies in the flood and survives only by clinging to small floating pieces of grass. The floodwaters carry Rikki-tikki into the middle of the people's garden. He is discovered by young Teddy, who thinks Rikki-tikki is dead. Teddy goes so far as to want to have a funeral for the mongoose, but Teddy's parents decide to try and revive Rikki-tikki. Fortunately, Rikki-tikki regains consciousness and makes himself an immediate part of the family.
He looked at the cotton wool, decided that it was not good to eat, ran all round the table, sat up and put his fur in order, scratched himself, and jumped on the small boy's shoulder.
Rikki-tikki curiously explores the house, and he decides to stay for a bit.
"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."
By the first night, Rikki-tikki has endeared himself to the family enough that they allow him to sleep on Teddy's pillow.