Totally engrossing, but (there's always a but) some quibbles. I think Jessica Raine was miscast; too young and inexperienced to be in charge of a 5 nation investigation into Romanian child trafficking. It also didn't help that she didn't project the authority and gravitas you would expect from someone in that position.
Also, wouldn't an old pro like Julien have wondered from the beginning why Martha would ask him to look into the "ordinary" case of a missing person. After all, at the time he was asked, he had no idea what it would lead to.
But, now a good but, the series otherwise kept me on the edge of my seat.