Judy and Peter work to expose and outsmart a group of real estate swindlers. The Piper family has won a piece of property in Roulsville which is 15 feet by 100 feet. Since the property is not wide enough for a house, Mrs. Piper must purchase the adjoining property at a much higher than usual price so that she can build a house. Peter is determined to help Mrs. Piper, so the young people design a home that can be built on a narrow lot of land and hire men to begin building the home. Soon after construction begins, the young people are warned to beware of the Red Circle. Strange sounds are heard at night as the Piper home is built. Several people become sick with a strange illness apparently caused by the Red Circle. Judy's search for the mysterious culprit becomes even more desperate when her beloved cat Blackberry falls ill!