“Drop whatever you're doing, Dr. Cross, and head to Reagan Airport,” DC Metro Police dispatch says. “A plane just crashed and exploded on the runway. The chief and the FBI want you and John Sampson there pronto.” The weapon that took down the plane is a remote-controlled Vietnam-War-era machine gun. The list of those who possess the training and expertise to operate the stolen, .50-caliber weapon is short. And time runs even shorter. As Cross and Sampson race to prevent another mass murder, their fearsomely armed opponent once again looks skyward.