Voice from the Cave by Mildred A. Wirt (Penny Parker #12)

VOICE FROM THE CAVE is #12 in the Penny Parker Series by Mildred Wirt Benson.

Penny is dismayed when her father's new friend, Mrs. Deline, travels with the Parkers on their vacation. Penny considers Mrs. Deline to be a pest and feels that Mrs. Deline has ruined their vacation. Fortunately, Penny is distracted by a search for foreign spies who are hiding in the area and have been transmitting information over the radio.