Ralph is a mouse who lives in the run-down Mountain View Inn, a battered resort hotel in the Sierra Nevada of California. Ralph longs for a life of danger and speed, wishing to get away from his relatives, who worry about the mice colony being discovered. One day a boy named Keith and his family visit the hotel on their way through California. Keith leaves a toy motorcycle on his bedside table, which interests Ralph. While Keith is away, Ralph attempts to ride it, but cannot figure out how to start it. Startled by a telephone ring, he and the motorcycle fall into a metal wastebasket. Keith discovers his missing motorcycle in the wastebasket along with Ralph and the two strike a friendship. Although Ralph's mother worries that he is in contact with humans, Keith shows Ralph how to start the motorcycle--make an engine-like noise--and lets Ralph ride it during the nighttime. While Keith and his family explore California, Ralph recklessly rides the motorcycle through the depths of the hotel. One night he is spotted by Keith's mother, who tells the management, resulting in a war on mice. Ralph and the motorcycle are almost sucked up by a maid's vacuum cleaner, but Ralph escapes, riding into a pile of dirty bedsheets. He escapes by chewing holes in the sheets, but, unfortunately, he must leave the motorcycle behind. After Ralph loses the motorcycle Keith loses trust in him, although he still brings the mice colony room service. One night Keith becomes very sick, and to regain Keith's trust, Ralph searches the hotel for an aspirin tablet, at risk to himself, for the medicine could prove fatal to a small mouse if ingested. His adventures include venturing outside the hotel, being trapped by guests, being chased by a nosy dog, and hatching a plan of adventure which involves a toy ambulance and the elevator. When Ralph succeeds, Keith's health is restored. He agrees to let Ralph keep the motorcycle, which is found by the bellboy, after he leaves, and Ralph uses the space under the TV set in the lobby to use as a garage; the motorcycle is his to keep.