With Lucas back within MI5 he reveals that the Russians interrogated him about Operation Sugar Horse, the most secret ongoing spy operation MI5 ever had. If the Russians know about it, then MI5 has a very high level mole. Harry seeks out his mentor, The Spycatcher, to help him find the mole. Ben is undercover with an Al Qaeda cell; they are planning to bomb London and what looks like a dry run unexpectedly turns live.
Elenco: Peter Firth , Nicola Walker , Matthew Macfadyen , Richard Armitage , Keeley Hawes , Hermione Norris , David Oyelowo , Max Brown , Lara Pulver , William Hope , Anna Chancellor , Sophia Myles , Natasha Little , Richard Harrington , Shauna Macdonald , Gemma Jones , Alex Lanipekun .