After an explosion destroys a building on Dr. Hugo Robbin's estate, apparently killing him, Ann Loring, the doctor's nurse, is arrested on suspicion of murder. At her trial, several children--Curley, Betty, Dudley, Ardda, Speck, Dis and Dat--attempt to gain access to the courtroom, claiming they have information relevant to the case. The bailiff throws them out, but while the defense attorney is questioning Ann, the children manage to find an open window through which they criticize her testimony. The prosecuting attorney then requests permission to have the children testify, and Ardda takes the stand and recalls what happened: Ardda is at Fixit-Dan's house having a doll repaired when Ann, who has claimed that she never left the doctor's on the day of the explosion, comes to see him. Dan has a back room where he is experimenting with a prototype atomic energy machine. Curley then takes the stand and continues the story: On the same day, Dan and the kids accompany him to Dr. Robbin's office to have his sore toe examined. When the doctor asks Dan if his experiments are successful, he replies that he is ready to build a full-scale machine. Dan is about to hand over the specifications and the all-important firing chamber to Dr. Robbin when he becomes suspicious of Robbin's intended use of the apparatus. They have an argument, which the kids overhear, and Robbin takes possession of the firing chamber, but Dan holds onto the plans. When Curley concludes his testimony, Dan suddenly shows up in the courtroom and the prosecutor orders him to be held for questioning. After the kids visit Dan in jail, bringing him keys and a book of fairy tales with a water pistol concealed in it, Dan tells his defense attorney that his firing chamber could not possibly have caused the explosion. As the chamber is a key piece of evidence, the kids decide to return to Dr. Robbin's sinister house to try to find it. There, they fall into an underground chamber, discover Robbin's research laboratory and find the firing chamber, only to have it taken from them by a chimpanzee. Ann and the defense attorney, who are also looking for evidence, then show up at the house to the house. Two of the kids, Dis and Dat, investigate an industrial-sized washing and ironing machine, which starts up with Dat inside, and the chimp puts the machine through additional cycles. After several "haunted house" adventures, Speck opens a door and the body of the attorney falls out. However, the lawyer has only been knocked out and he and Ann continue their own search. The chimp eventually surrenders the firing chamber to Dudley and Speck, but they are then trapped by a gorilla. After Curley finds a phone and summons the police, "Rags," the kids's dog, helps Dan to escape from the jail. They arrive at the house as the kids are being pursued by the giant gorilla. The police shoot the gorilla, who turns out to be Dr. Robbin in a gorilla suit. Ann explains that she was suspicious of what Robbin was doing and speculates that he must have faked his own murder in an attempt to incriminate Dan and her.