Frank and Joe Hardy are on a trip to Arizona's magnificent Organ Pipe Cactus National Monument with father. There they meet their father's old friend Winston Grisham, or Grish, as he liked to be called. Grish takes the Hardy's on a tour showing them around the park. Frank spots big tire marks and questions Grish why there are big tire marks. Grish tell them that there are cactus thieves. So there is another case for the Hardy's to solve. Frank and Joe decide to help Grish. Grish does not agree at first, but afterwards hi agrees. Fenton Hardy invites Grish for dinner. Then they see a blue pickup truck coming their way. They all manage to escape it. The Hardy's ask Grish who it was. Grish tells them that it was David Kidwell. Grish yells at Kidwell and asks him why he did that. Kidwell replies that he came to get his shovel.
The Hardy's introduce themselves too Kidwell. Kidwell apologizes to Grish and tells him that he will talk to him inn the morning. Then he rides away. Kidwell becomes the first suspect of the Hardy's. Grish said that he has the background and skills to be able to dig up a large plant. Grish told us that most of the campers stay for weeks or even months. He told the Hardy's about a professor named Townsend and his daughter Diane. They were doing some biological research. He drove away. Grish comes for dinner. After dinner Joe and Frank take a stroll while Fenton and Grish talk. Frank and Joe see somebody in the dark. Finding out that he has been noticed, he flees. But Frank and Joe catch up with him and ask him who he was. He tells them that he is Raymond Perez...