In the latest in this USA Today bestselling series, Nevada sheriff Porter Beck receives an anonymous riddle leading him to a body hidden for decades, leading Beck and his team in a chase to unravel riddles that will help them uncover and stop a long-time serial killer.

Porter Beck is the sheriff of Lincoln County, Nevada, which is mostly desert and mountains, large in area and low in population, largely known to the outside world for containing Area 51. It is there, in the Black Mailbox near Area 51, that a communique for Beck himself is found. It's a riddle that, once deciphered, leads Beck and his team, as well as Las Vegas PD, to Lake Mead where, in its ever-shallowing waters is found an old oil drum chained to the bottom of the lake. Inside are human remains, which prove to be Beck's maternal grandfather, missing since the 1970s.

The case brings out an FBI profiler who believes this riddle ties to a previous case, the disappearance of her own brother seventeen years earlier, and a riddle she received that she could never decipher.

Now convinced they are on the trail of a serial killer - someone who sends riddle after riddle, sending Beck and his team from the deserts of Nevada to the mountains of Arizona, and ultimately to eastern Utah in order to uncover and stop a killer from a last, brutal murder.