This is The Guardian.

We're bringing you some of our favourite episodes from the past year.

On Friday, we told you about a video of a secret war crime committed in the early years of the Syrian Civil War.

The footage was leaked to two researchers who spent years investigating it,

until they found the man responsible. And to their surprise, when they sent him a message,

he called them.

When you saw that he was calling you, what did you think?

I was very excited.

I wanted to see the guy to confirm that's Tim. Who is it?

How he speak. What he's gonna tell us.

And so did you answer?

Who is it?

And I smiled.

Hello.

What's up?

What's up?

I'm glad.

Today, how those two academics talk their way into the heart of one of the darkest corners of the Syrian government,

and what they found hidden there.

Before we start, heads up.

This episode contains graphic descriptions of violence,

if you're feeling in any way sensitive, especially with everything going on in the world right now.

...