Dan Holiday, a mystery novelist who advertises for adventure, receives a letter from a young boy named Marty Kennedy. Marty is worried about his friend Ted Whitman, an astronomer working at the Williams Observatory atop a mountain. Ted usually signals Marty using a mirror, but he hasn’t done so in days, leading Marty to suspect something is wrong. Holiday travels to Collingwood and meets Marty, who insists they speak in secret. Marty explains that Ted has been missing and asks Holiday to investigate. When Holiday arrives at the observatory, he is met by a man claiming to be Ted Whitman—but something feels off. Soon, Holiday realizes he has stumbled into a dangerous situation involving criminals hiding out at the observatory. The criminals, led by a man named Frank, are using the observatory as a hideout after a jewelry store robbery. They have taken Ted Whitman hostage and are preventing him from getting medical help for a gunshot wound. Holiday and Professor Irving, another astron...