April 6, 1967. Seaman Apprentice Doug Hegdahl, 20 years old, a farm kid from South Dakota, is on deck of the cruiser USS Canberra in the Gulf of Tonkin at night when a 5-inch gun fires. The blast knocks him overboard. He treads water for hours until Vietnamese fishermen pull him out. He ends up in the Hanoi Hilton. His interrogators can't believe a sailor just fell off a ship, so they assume he's a commando. Hegdahl figures out his best chance is to act dumb. He plays the bumpkin. He pretends he can't read or write. They assign a guard to teach him. He never learns. They give up and nickname him "The Incredibly Stupid One." Then they let him wander the camp almost freely. It was the worst mistake they ever made with him. With an Air Force officer coaching him, Hegdahl memorized the names, capture dates and personal details of about 256 fellow prisoners. He locked them in his head by singing them to the tune of "Old MacDonald Had a Farm," over and over. He talked the guards into taking him into Hanoi for new glasses and memorized the route. He also put dirt in the fuel tanks of five North Vietnamese trucks. The POWs had a rule: nobody accepts early release. They made one exception. When Hanoi offered to free Hegdahl for propaganda in 1969, his senior officer ordered him to go. He walked out with 256 names in his head. For hundreds of families, it was the first proof their men were alive. The North Vietnamese had sent home the most dangerous man in the camp.