World War II is in progress, and mathematics professor Rigby Webb receives a mysterious call to take a job with a physician, Duncan Chandler, in rural Pennsylvania. He heads there as a major snowstorm sets in.

Chandler, also an expert on codes and ciphers, has turned his home medical office into a secure room to work on a secret government project - creating an unbreakable cipher for war communications. Webb is also an expert on codes and ciphers. Webb accepts the job offer as his assistant. His previous assistant met with an "accident" which no one talks about.

One of the guests is bitter Max Harwick, a famous pianist, who has just lost two fingers after a hunting accident. Chandler had to amputate them to save his life, but Harwick's career is ruined. Attorney Ross Harwick, his son, wants an out-of-court settlement, but Chandler insists on going to trial to prove his good intentions.

Word comes that an enemy spy is after the secret cipher. A courier is coming to the house with news of his identity. As he approaches, a shot is fired from an upstairs window, killing the courier and causing him to fall into an abandoned quarry.

With the house firmly snowed in, one of the occupants is likely the enemy spy - and a murderer. RM