第三季改编自原著系列中的第三本《猛虎（Real Tigers）》。
　　伊斯坦布尔的一名多情联络员威胁要曝光伦敦方面隐瞒的一个军情五处秘密。当Jackson Lamb和他那群格格不入的手下被卷入这场战斗后，他们发现自己陷入了一场阴谋，这场阴谋不仅威胁到Slough House的未来 ，也威胁到了军情五处本身的未来。

　　Ross (Turner) must defend Cornwall from an empowered George Warleggan (Jack Farthing), and risks everything he holds dear as he embarks on a political journey which takes him to the nation's capital. Demelza (Tomlinson) finds her loyalties torn, Elizabeth (Heida Reed) tries to strengthen her marriage, Morwenna (Ellise Chappell) continues to be oppressed by husband Reverend Ossie Whitworth (Christian Brassington), and the Enyses (Luke Norris and Gabriella Wilde) are tested as never before.