Twenty minutes after Sgt. Bowe Bergdahl walked away from his remote Army outpost in Afghanistan in the middle of a summer night, carrying little more than vacuum-packed chicken, knives, water and a compass, he began to realize just how dire his predicament was.

“I’m going, ‘Good grief, I’m in over my head,’ ” Sergeant Bergdahl said.

“Suddenly, it really starts to sink in that I really did something bad,” he continued. “Or, not bad, but I really did something serious.”

Sergeant Bergdahl recounted his experience publicly for the first time in the premiere episode of the second season of the podcast “Serial,” which was released at 6 a.m. Thursday. In interviews with the screenwriter Mark Boal, he explained in his own words why he had left his base in June 2009, an action that prompted a manhunt involving thousands of troops and led him to spend nearly five years in brutal captivity under the Taliban.