In Season 4, Double Agent, Gabby Lane is sent to Frankfurt to collapse a terrorist bank. When a rogue British spy goes missing after the murder of a Germany politician, she discovers that the murder and the bank are linked to a major crime network, run by Oskar Zetkin, who will do anything he can to stop Gabby's mission. 
But who is the real double agent?
As loyalties and ambitions clash, more deaths occur, and the guilty need to choose whether to fight or run, and a chase to the death begins across the snowy peaks of Switzerland.