In 1969, newspapers in Northern California began receiving letters from a killer who called himself Zodiac. Some contained cryptograms, threats, and details intended to prove that the writer had knowledge of the crimes. One cipher was solved almost immediately, while another resisted serious attempts at decipherment for more than fifty years.

This episode examines what the Zodiac letters actually revealed, the breakthrough that finally unlocked the famous three hundred forty character cipher, and why solving a message did not solve the identity of its author.

The code breakers eventually found the words hidden inside one of America's most famous ciphers. But the name everyone wanted was not there.