Judy Bolton and Lorraine Lee's double wedding day has finally arrived. It proves to be a day of surprises, for Roberta suddenly arrives at the Bolton home, warning Judy not to open a wedding present sent to her by Dale and Irene Meredith. Judy assumes that Roberta is telling stories since she claims that the present is a radio that will explode, and surely Dale and Irene would do nothing to harm Judy. Shortly after the wedding, Roberta is seen running into the woods with the package, and then an explosion is heard. Apparently the radio was indeed wired with an explosive device. Fortunately, Roberta is unharmed and is later dropped off at her home by Judy and Peter. Mysterious events follow Judy and Peter at each place they stop during their honeymoon vacation. They discover that the Griggs are involved in the plot and that they are not actually related to Roberta. The criminals are brought to justice, and Roberta comes to live with Peter and Judy.