This is the Guardian.

Over the past week, we've been bringing you some of our favourite episodes from this year.

Today, the first of a two-part investigation, we first broadcast in April,

about two ordinary people who find evidence of a war crime,

and then decide to go undercover to find the man responsible.

Before we start, a heads up.

This episode contains extremely graphic descriptions of violence.

If you're feeling in any way sensitive, especially with everything going on in the world at the moment,

you might want to save this and come back to it another time.

A few years ago, somewhere in the Syrian capital, Damascus, a laptop broke down.

This particular laptop belonged to an office of a Syrian intelligence agency,

called Branch 227.

Good IT support is hard to find.

One of the commanders of this unit says,

he's a young recruit to a militia working with the Syrian army and he's good with computers.

But what the commander didn't know was this young guy, he was also filled with doubts.

Doubt that he's on the right side of this civil war that's been engulfing Syria since 2011,

whether he's doing the right thing, fighting on the side of the government.

And when he starts looking through the laptop, trying to fix it, he says something, a video.

And even in a country where the government has leveled cities to the ground,

killed hundreds of thousands of civilians and disappeared tens of thousands of others.

What he sees in that video, leaves him astonished.

...