Sometime in the early morning hours of April 6, 1967,[2] 20-year-old Hegdahl was onboard the USS Canberra in the Gulf of Tonkin, three miles off the coast, when he was knocked overboard by the blast from a 5-inch gun mount.

He reported that fellow U.S. captives in the Vietnam War would authenticate a new prisoner's U.S. identity by using "Shave and a Haircut" as a shibboleth, tapping the first five notes against a cell wall and waiting for the appropriate response.

According to his senior officer and cellmate, Lieutenant Commander Richard A. Stratton, Hegdahl also convinced his captors that he needed new glasses and memorized the route from the prison into the city of Hanoi, where he was taken to be fitted.

[9] Hegdahl was one of three POWs (along with Navy Lieutenant Robert Frishman and Air Force Captain Wesley Rumble) who were released on August 5, 1969, as a propaganda move by the North Vietnamese.

[7] He was ordered to accept an early release so that he could provide the names of POWs being held by the North Vietnamese and reveal the conditions to which the prisoners were being subjected.

After his discharge, Hegdahl was sent to the Paris Peace Talks by Ross Perot in December 1970 and confronted the North Vietnamese with his first-hand information about the mistreatment of prisoners.

[10] After returning to the United States, Hegdahl used his experiences as an instructor at the U.S. Navy's Survival, Evasion, Resistance and Escape school at NAS North Island, San Diego.