Amid the more than 30,000 defections to South Korea from North Korea, Lee Soo-keun’s stands out as one of the most sensational and tragic.

He was welcomed as a hero in South Korea in 1967 after he escaped over the border under a hail of bullets and with the help of American soldiers. Two years later, he was caught trying to leave the South on a fake passport and charged with spying for the North. Enraged South Koreans burned him in effigy, and he was swiftly convicted and hanged.

This month, nearly a half-century after his execution, Mr. Lee’s story took another dramatic turn: A court in Seoul, the South Korean capital, absolved him of espionage, ruling that he had been wrongfully executed based on fabricated charges and a confession obtained through torture.