This story exists in many forms and is told all over Japan. There are alternate endings which I will cover below.
The main story here is largely borrowed from this blog, and was translated by Sarah Brumagin (who kindly agreed to help speed me along — many thanks!).
Once upon a time there lived a lonely peddler.
One day, passing through a mountain, a lone wolf appeared.
The peddler was scared stiff, but the wolf showed no sign of attacking.
Instead, it opened its mouth wide and let out a pained whine.
The peddler timidly peered into the wolf’s mouth and saw a bone lodged in the back of its throat.
The peddler took pity on the wolf and removed the bone.
The wolf wagged its tail happily.
From that day forward, the wolf always followed the peddler on his journeys back and forth over the mountain.
Thanks to the wolf, the peddler saw neither hide nor hair of any beasts or bandits and he always crossed the mountain safely.
In other variations, the peddler awakens in the morning to hear the sound of something heavy being dropped at his door.
He goes to check and sees the wolf has dragged a dead body to his house as a gift. He says, “I don’t eat humans.” And the wolf drags the dead body away.
The next morning he hears something else being dropped and goes to check. This time it is a dead rabbit. He thanks the wolf and the wolf returns to the mountains having repaid its debt.