Part 1

Ministry of Interior's Paul Frederick wants to use Robert Sadler, awaiting death following a government show trial, to infiltrate terrorist groups and get the names of the leaders.

Part 2

Determined to find the evidence against his wife's former mentor Professor Donati, Paul is willing to go to extremes, even distorting information to make Donati look guilty of conspiracy.

Part 3

Paul tells Donati he has brought forward the execution of Sadler, convinced he will now break cover and finally give up his own wife as a traitor.