Karl and Elise reunite for an emotive and morally complex investigation when a boat carrying child refugees is torched in the middle of the Channel, leaving no sign of its passengers.
Karl and Elise try to decode the Pied Piper’s demands, while BB joins the French investigation in a race against time to find the perpetrators before they strike again. Meanwhile, Charlie Moreau returns to his family, with terrible consequences.
Another message from the Pied Piper leads Karl and Elise to a horrifying discovery. As the net closes in, one of the cornered perpetrators lashes out - could this be the end of the line for one of the team?
When Karl and Elise reach a dead end in the case, they both resort to unconventional means in pursuit of the truth. With the body count rising, the stakes could not be higher - but will their gambles pay off?
An increasingly isolated Karl makes a major breakthrough in the search for Saban – but his sense of triumph is short-lived when he discovers that someone close to him is in mortal danger.
The final episode of The Tunnel sees Karl and Elise face a tense and gripping showdown with the Pied Piper. There is also closure at last for Charlie Moreau, as Karl and Olivier uncover the truth about his abduction.