Spooks (2002)

S7E3 The Tip-Off

Now that Lucas has returned to MI5, he discloses that the Russians had interrogated him regarding Operation Sugar Horse, the agency's most classified mission. This revelation raises alarms for Harry, as it indicates the presence of a serious mole within MI5. To tackle this threat, he turns to his mentor, The Spycatcher, for assistance in identifying the traitor. Meanwhile, Ben is operating undercover within an Al-Qaeda group that is plotting a bombing in London, and what initially appears to be a dry run suddenly escalates into a real crisis.