When Sergei and Yulia Skripal were poisoned with a nerve agent, police faced one of the most complex investigations in their history. They had to trace a weapon which they could neither see nor smell, but it could kill thousands. In this episode, we hear how they checked everything from coins to car keys in the hunt to find the source of the poisoning.

This episode was made by Marie Lennon, Andy Howard, Tom Ryan & Dan O’Brien.

Executive production by Mary Sanders and Naomi Wordley.