On a sunlit morning in September 1978, a sloop drifts aimlessly across the Chesapeake Bay. The cabin reveals signs of a struggle, and “classified” documents, live 9 mm cartridges, and a top-secret “burst” satellite communications transmitter are discovered aboard. But where is the boat’s owner, former CIA officer John Paisley? One man may hold the key to finding out. Tennent “Pete” Bagley was once a rising star in America’s spy aristocracy,...