The episode begins with a demonstration of the most advanced military robot ever built: the Unitron, a virtually indestructible super-tank. It can be controlled by a human operator or programmed to attack a designated target until it is destroyed.
In a transmission to Earth, the Mysterons warn Spectrum that they intend to assassinate the Supreme Commander of Earth Forces. Spectrum commander-in-chief Colonel White assigns Captains Scarlet and Blue to protect the Commander. Meanwhile, two Earth Forces officers, Colonel Storm and Major Brooks, are killed in a road accident and reconstructed by the Mysterons to carry out the threat. At the Supreme Headquarters Earth Forces (SHEF) building in New York, the Commander, joined by the Spectrum captains and the reconstructed Brooks, chairs a press conference unveiling the Unitron. The Mysterons make an attempt on the Commander's life by turning Brooks into a living bomb. However, they are thwarted when Scarlet, who has a "sixth sense" for Mysteron activity, activates emergency barriers that shield the Commander, Blue and himself from the explosion.
Later, Blue takes the Commander to Point 783, a military blockhouse on the Unitron's test range in the Sahara, to view the weapon in action. Also in attendance is the reconstructed Storm. At first the demonstration proceeds according to plan, with the Unitron efficiently destroying its pre-ordained targets. However, when the Commander steps outside, it turns its fire on Point 783 itself. Repeated aerial bombardments by the Spectrum Angel squadron fail to stop the Unitron.
Scarlet requisitions a hidden Spectrum Pursuit Vehicle from a desert bazaar and proceeds to Point 783 to extract the Commander, who is left in Storm's charge while Blue and the other personnel remain in the blockhouse. Scarlet, Storm and the Commander speed away in the SPV just as the Unitron closes in on Point 783. However, moments before the blockhouse is destroyed, the Unitron abandons its assault and heads off in pursuit of the SPV. At this point it is revealed that Storm has re-programmed the Unitron – and that its target is not Point 783, but Storm himself. In the SPV, Storm repeatedly shoots Scarlet, but despite his fatal wounds the captain is still able to eject both himself and the Commander. The SPV, with Storm inside, is pursued by the Unitron until both vehicles are destroyed when they plunge over a cliff.
The Commander is rescued by Spectrum while Scarlet is taken away to recover from his injuries. At the end of the episode, Blue assures the Point 783 personnel that Scarlet will return to fight the Mysterons again."
Major Brooks becomes a living suicide bomber like Captain Brown in the first episode. Pretty graphic for a kids show, with body parts blowing everywhere. Talk about Mortal Kombat for the 60s!
BTW after what happened to Brooks, didn't anyone think Colonel Storm could have been a Mysteron as well?
It'd have still made for a dramatic end with Scarlet driving the SPV towards the cliffs and eject the Supreme Commander at the last minute.