Mr Hoppy is a very shy old man who lives alone in an apartment building. For many years, he has been secretly in love with Mrs Silver, a woman who lives below him. Mr Hoppy frequently leans over his balcony and exchanges polite conversation with Mrs Silver, but he is too shy to disclose how he feels. Mr Hoppy longs to express his feelings to Mrs Silver, but he can never bring his lips to form the words.
Mrs Silver has a small pet tortoise, Alfie, whom she loves very much. One morning, Mrs Silver mentions to Mr Hoppy that even though she has had Alfie for many years, her pet has only grown a tiny bit and has gained only 13 ounces in weight. She confesses that she wishes she knew of some way to make her little Alfie grow into a larger, more dignified tortoise. Mr Hoppy suddenly thinks of a way to give Mrs Silver her wish and win her affection.
Mr Hoppy tells Mrs Silver that he – in fact – does know of a way to make a tortoise grow bigger. He writes the following words on a slip of paper, and lowers it down to Mrs Silver:
Mr Hoppy explains that these magic words, when whispered in Alfie's ear three times a day, will cause Alfie to grow bigger and bigger. Mrs Silver is doubtful, but agrees to try. (The words are, reversed, Tortoise, tortoise, get bigger bigger! Come on, tortoise, grow up, puff up, shoot up! Spring up, blow up, swell up! Gorge! Guzzle! Stuff! Gulp! Put on fat, tortoise, put on fat! Get on, get on, gobble food!)
Over the next few days, Mr Hoppy carries out the second part of his plan. He visits every pet shop in the city, and buys many tortoises of various sizes, but none that weigh less than 13 ounces. Mr Hoppy brings all the tortoises back to his apartment and installs them in a makeshift corral in his living room. Next, Mr Hoppy builds a special tool (using his mechanic's expertise from his days working at a bus garage) to help him snatch the tortoise from Mrs Silver's balcony. He fastens a handle to the end of a long metal tube, and a tiny claw at the bottom. By pulling the handle, the arms of the claw gently open and close.
The following day, when Mrs Silver leaves for work, Mr Hoppy selects a tortoise from his living room that weighs exactly 15 ounces. He carefully picks Alfie up from the lower balcony, and exchanges him with the new tortoise. When Mrs Silver returns home, she faithfully whispers the magic words in Alfie's ear, but does not notice that an exchange has been made.
Over the next 8 weeks, Mr Hoppy continues to switch Mrs Silver's current pet with a slightly larger tortoise, but she still does not perceive that her pet is growing in size. One afternoon, Mrs Silver comments to Mr Hoppy that Alfie seems a bit bigger, but cannot tell for sure. Suddenly, Mrs Silver notices that Alfie can no longer fit through the door to his house, and exclaims to Mr Hoppy that his spell is sure to be working. Mrs Silver runs inside and weighs her pet, and is surprised to find that Alfie now weighs 27 ounces, more than double the weight he was before. Mr Hoppy summons his courage and asks Mrs Silver if he can come down and see the effect for himself. Mrs Silver, in raptures over her pet's transformation, gladly grants his request.
Mr Hoppy races down the stairs, nervous and excited to be on the brink of winning Mrs Silver's heart. Mrs Silver flings open the door, embraces Mr Hoppy, and expresses her admiration for Mr Hoppy's magical spell. However, the tortoise cannot fit in the house now, so Mr Hoppy tells Mrs Silver to say a magic spell that night and the next night, secretly replacing this tortoise with one slightly smaller during the day. His part works splendidly, and Mr Hoppy, suddenly emboldened by Mrs Silver's warm smile, asks Mrs Silver for her hand in marriage. Mrs Silver delightedly accepts Mr Hoppy's proposal, then adds that she thought he would never get around to asking.
Mr Hoppy secretly returns all the tortoises in his living room back to their respective pet shops, and Mr Hoppy and Mrs Silver are happily married a few weeks later. And Alfie gets bought by a girl called Roberta Squibb, and grows bigger.