Tom Buckingham (Sam Heughan), a suspended special forces operator, is taking Dr Sophie Hart (Hannah John-Kamen) from London to Paris to propose. When their train is deep inside the Channel Tunnel, heavily armed mercenaries, led by Grace Lewis (Ruby Rose), seize control of it and hold everyone hostage. Grace threatens to blow up the Channel Tunnel and declare economic war on a government that has its fair share of secrets to keep. Unarmed and cut off from his counter terror team, Tom is the only hope that Sophie and the other passengers have to make it out alive. To save them he must embrace the unique psychology that makes Grace such a formidable adversary.