Right after Sept. 11, the United States created a brand-new criminal justice system at Guantánamo Bay. It was a prison and a court designed to deal with the people we had captured whom we suspected of being members of the Taliban or al Qaeda.

But to do what we wanted to do at Guantánamo — to interrogate detainees the way we wanted, to hold them indefinitely without charging them with a crime — we had to push aside the old, time-tested rules for detaining prisoners of war. And the consequences of that fell on ordinary people: thousands of military personnel, hundreds of prisoners, everybody scrambling through the same experiment.

There has been great journalism about the legal maneuvering to justify Guantánamo, and about the detainee abuse and the politics and policy. But “Serial” reporters Sarah Koenig and Dana Chivvis were after the inside stories, a picture of Guantánamo you could get only from the people who went through it. For years, though, all the best stories they heard about Guantánamo were off the record. But they stuck with it, figuring maybe once enough people were back in civilian life they’d be willing to tell those stories on the record. A couple of years ago, the “Serial” team started contacting people again: guards, interrogators, commanders, lawyers, chaplains, translators and former prisoners. More than a hundred people. And a remarkable number of them said: Okay, I’m ready. Here’s what happened.

“Serial” Season 4 is a history of Guantánamo told by people who lived through key moments in its evolution, who know things the rest of us don’t about what it’s like to be caught inside an improvised justice system.