6th form students find a TV show interrupted by the "Voice of Truth". Martin Clifford is approached by Forrester warning of a traitor at Radlow School and needs his help. Mr. Brooks shows Martin how it was broadcast but is later attacked.
Headmaster Mr. Brooks is found badly injured in the pigeon loft and dies on the way to the hospital. Spud develops his negatives, Martin spots Forrester in the loft. He is attacked, the pictures ruined and a warning is sent in the post.
Forrester persuades Martin to aide his investigation by showing him his evidence. The government believe there is a spy trying to access Redlow US Airbase. Mr. Elliott learns Special Branch are inquiring about him so burns his passport.
Redlow US Airbase is on high alert and Martin's dad is arrested and held in custody. Martin finds a discarded carrier pigeon message containing the formula for "Mercury Carbonate". The same chemical that Miss Walker left on the blackboard.
Martin learns his father was arrested when four West German Star Fighter jets were sabotaged at the airbase. Jane Walker is getting information from a sergeant at the base, coding and putting it on the blackboard but who is her connection?
Martin accuses Mr. Harvey of being the Voice of Truth" but instead learns he and publican Ron Desmond are British Inteligence. They worked for Mr. Brooks and never heard of Forrester. Jane Walker blackmails Mr. Elliott to spy on Martin.
Martin's father admits he was bribed to get access the base. The 'Voice of Truth' gives a last message "Courage, Strength and Truth". Martin escapes death in the steam room but is shocked to learn Forrester arrive has a visiting lecturer.
Forrester claims his British Inteligence code name "Uncle John" and sends Martin to a derelict farm, unaware that someone is transmitting from a radio. Jane Walker is planning to leave but youngsters have set up a camp near her home.
Forrester informs Martin that Harvey is a double agent. Martin had seen a transmitter in the home so is sent back but finds Sgt. Sikowski installing a bug. Jane Walker has rigged Harvey's telephone, when Sikowski picks it up, it explodes.
Jane Walker uses a truth drug on Harvey and makes Elliot search for Forrester. With Harvey's name cleared, Martin is used has bait in a plan to catch Walker. Elliot is allowed to flee but has to bring Jane Walker into Forrester's hands.
