Episode 4

The World Trade Summit for Africa is meant to be the hopeful one: a global conference full of delegates and good intentions, the kind of event that could signal the end of poverty on the continent. Undercover among the delegates, MI5's team is there to make sure the hope is not a target. They find two conspiracies instead of one, each threatening lives, and neither stopping at the conference doors.

The first is an assassination plot, clear enough that the team's mandate starts out simple: get in the way, stop the murder. Then the motive surfaces, and simple evaporates. The team, initially ready to put their lives on the line to prevent the killing, is forced to think again, and the thinking is not comfortable. The dilemma stops being about how to stop the killing and starts being about whether it should be stopped at all. It is a moral dilemma with a deadline attached.

George Harris's Gabriel Sekoa and Alex Jennings's James Allan give the politics a human face, while Nikki Amuka-Bird's Michelle Lopez and Gugu Mbatha-Raw's Jenny keep the operation honest from inside. The episode trusts its audience to sit with the discomfort instead of resolving it neatly.

David Farr and David Wolstencroft wrote it, Kenneth Glenaan directed. Peter Firth's Harry, Raza Jaffrey's Zaf, James Dicker's Wesley Carter, Thomas Aldridge's Phillip Patel, Lucian Msamati's Manu Buffong and Louise Minchin's Newsreader complete the cast. It aired in October 2006.

But the real subject is the one nobody on screen will say out loud: whether a good deed done for the wrong reasons is still a good deed. The summit's stage is global, but the choice facing the team is entirely personal, and the gap between the two is where the episode lives. There is no tidy exit, and the episode does not manufacture one.