After a body falls from the 59th Street Bridge, the CSI team finds a compass near the body pointing east, and realizes it's the third victim of the Compass Killer. Meanwhile, detective Flack, still dealing with Angell's death, goes AWOL.

Mummified remains are found in a booby-trapped Manhattan penthouse that has been closed off for more than 80 years, and Mac and Stella race against time to find the people who discovered the body, who are still trapped in the building.