A new hunt for the mysterious Zodiac Killer gains unprecedented access to files, unearthing a complicated cypher that may contain the killer’s name, and links between the Zodiac Killer and an unsolved case. A prime suspect with code training emerges.

The team uncovers clues that the killer may have served in the U.S. Armed Forces, as the supercomputer identifies symbols in the code that would have been known to only a few when it was written; Sal and Ken find new evidence.

The team uncovers a mysterious new cipher related to the Zodiac Killer, while the supercomputer uncovers a strange lunar pattern linking the killings; a taunting riddle by the killer may lead to a body; a second prime suspect emerges.

The hunt goes into high gear as the FBI releases secret files on the killer; a handwriting analyst connects the Zodiac's letters to a taunting letter mailed 3,000 miles away in Albany, N.Y.; the code team has a massive breakthrough on the Z-340.

Sal and Ken investigate San Francisco, the epicenter of the Zodiac Killer murders; the Code Team announces a team member has solved the Zodiac's infamous Z340 code, which has eluded codebreakers for 50 years.