 Dennis and Tommy try and sell Valentine s day cards in July to buy swim fins. However, it isn t long before chaos hits the neighborhood as it causes problems for Miss Cathcart, Mr. Dorfman, Mr. Wilson and even the gardener Mr. Carlson. There also ends up being a run on Valentine s day cards in July. Mr. Wilson buys the rest of Dennis  cards hoping to avoid any more trouble but it ends up with Mr. Wilson in trouble with the police as they think he was the one responsible for all the trouble with the Valentine s day cards.