Spectrum Strikes Back

Colonel White, Captain Scarlet and Captain Blue go to the headquarters of the Spectrum Intelligence Agency, where a scientist shows them and the World President the new advances made, following the events of 'Operation Time'. The SIA have developed a Mysteron gun that emits a beam of electrons able to kill a Mysteron from fifty yards. Also, a Mysteron detector has been developed, using X-rays.

The scientist asks Captain Indigo, who is currently being Mysteronised by Captain Black, to bring the detector down. He does so, and the scientist demonstrates negative results by using Colonel White and the World President as subjects, and then a positive result using Captain Scarlet. The World President asks if he can try it, and he takes a picture of Captain Indigo, who quickly returns upstairs to keep an eye on things.

When the developed picture shows a positive result, Captain Scarlet grabs the Mysteron gun and sets off after Indigo, but Indigo has triggered the mechanism that lowers the hunting lodge to the level of the headquarters, and taken the key with him. Scarlet finds Indigo and kills him with the Mysteron gun in time to return the key to the hunting lodge and save everyone trapped below.