After being knocked off his bike, Martin Clifford arrives at school late but unhurt. He is just in time for a television broadcast, which is suddenly interrupted by a voice urging students to question what purpose their school serves. Martin is later confronted by Forrester, the driver of the car that struck him, whop tells him that someone at the school is a traitor...
Martin examines the television to try to find the source of the broadcasts, but his search is interrupted by chemistry teacher Jane Walker. Having dismissed him, she copies a chemical equation onto the blackboard. The mystery deepens as Martin and Spud develop photographs taken days earlier, and a previously unseen figure emerges in the background of each picture.
Martin wakes to find himself in a four-poster bed in a huge baronial hall, and Forrester explains that he shot him with a tranquillizer gun. As Martin's father and Spud anxiously try to trace him, Forrester explains that 'The Voice of Truth' is the work of enemy agents. He equips Martin with a spy kit, and instructs him to fail his approaching 'A' levels.
Martin and Spud examine the slides, and see various people hidden at the end of the runway at the air base - among them Elliot and Ron Desmond, landlord at the Red Lion. Elliot tells Jane how he came to be in Britain, and the following morning, as Forrester questions his father, Martin makes an important discovery in the pigeon loft.
At the air base, Martin hides and is suprised to be joined by Forrester, who later tells him that his father is suspected of attempting to sabotage four fighter aircraft. Jane Walker's role becomes increasingly clear, and Martin locates the source of 'The Voice of Truth'.
Martin is interrogated by Harvey and Desmond, who tell him they are British intelligence agents - and have never heard of Forrester. While Jessie and Forrester attempt to decode the chemical equation, Jane blackmails Elliot. She tells him that no harm will come to his family if he co-operates, and orders him to trail Martin.
Martin, left lying in the road after jumping clear of the speeding car, is helped up by his father, who reveals exactly why he was beaten up. On the last day of the school term, Martin learns to his horror that next term will bring a new headmaster: Forrester!
Martin demands an explanation from Forrester. Clifford and Jessie come face-to-face with the two pursuers, while Jane is dismayed to find a group of guides and scouts camping on land at her farm, from where she is sending coded messages. Martin is sent to investigate further...
Martin is introduced to his young bodyguards, while Harvey, tricked into a farewell drink with Jane, now lies unconscious as she continues to communicate with 'The Voice'. Forrester convinced Harvey and Jane are working for the Russians, instructs Martin to search Harvey's home; it proves a highly dangerous assignment.
Sikowski regains consciousness at Harvey's home, and gives Martin his wallet. Martin overhears Jane, now known to be a Russian agent, tell Elliot she has another job for him: locating Forrester. Martin rushes to the shop, where he alerts Forrester, and they examine the wallet. It contains Pentothal - the truth serum.
The captive Martin overpowers his guards at Jane's house, but senses he is being watched from behind a two-way mirror as he searches her home; he suspects there is a room behind it. When the phone rings, he answers it to hear a voice he recognises as Desmond's. Has he identified the double agent?
Harvey, suffering from amnesia, visits Jane once more, and is disturbed to find the room changed and the mirror gone. Fletcher senses that Martin deliberately failed his exams, and asks Clifford who might have instructed him to fail, while Forrester returns to search Jane's chemistry room. She is prepared for him.
Aboard the Russian trawler, Forrester suddenly realises that Jane is the infamous Major Sverdlow. On their return to Redlow, he and Martin discover that Desmond has been caught; on searching Jane's home, police find a tape recording of 'The Voice'. It seems Martin is finally to discover who is behind the broadcasts.