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While relaxing at a smart countryside restaurant during a late evening, Captain Blue, accompanied by Captain Scarlet, starts to feel strange after drinking a cup of black coffee.

When Scarlet issues himself to fetch their coats, Blue instantaneously disappears, and when awakening, finds himself sitting in Lieutenant Green's chair back on Cloudbase, with a Spectrum Intelligence agent, who calls himself Colgan, sitting in Colonel White's chair.

He explains that Blue has been absent without leave for three months, and must prove his identity by disclosing top-security information; the Spectrum cipher codes, before he can return to active duty.

Without having to do this, Captain Blue instead tells the story of three Spectrum assignments involving the Mysterons: the nuclear attack on London, the mission of destroying the Mysteron complex in Crater 101, and the prevention of a Mysteronised Commodore and his aid from slaughtering the Supreme Command of the World Air Force at the Scottish Glen Garry Castle, which wasn't released to the press.

But Colgan still continues to question Blue regarding the cipher codes, whom hasn't caught on the fact that this mysterious S.I. officer is truthfully a Mysteron agent, and the Cloudbase interior a replica of the original, located in an abandoned warehouse.