Set in 2068, the series depicts a "war of nerves" between Earth and the Mysterons: a race of Martians with the power to create functioning copies of destroyed people or objects and use them to carry out acts of aggression against humanity.
Earth is defended by a military organisation called Spectrum, whose top agent, Captain Scarlet, was killed by the Mysterons and replaced by a reconstruction that subsequently broke free of their control.
On Cloudbase, Spectrum learns the target of the Mysterons' latest threat: the Najama complex, an automated facility in the Andes that desalinates seawater to irrigate the interior of South America.
Colonel White dispatches a team of field agents, led by Captains Scarlet and Blue and accompanied by the Angel squadron, to the Najama Valley to set up surveillance posts around the complex.
Shortly after Spectrum's arrival, Captain Black slips into the ancient Aztec temple where Scarlet and Blue are based and conceals a radio transmitter in the mouth of a statue of the Sun God.
Realising that they cannot locate and destroy the transmitter in the time remaining, they evacuate the temple in their Spectrum Pursuit Vehicle and order the Angels to launch an aerial assault on the building.
[7] In a mostly negative review, writer Fred McNamara criticises the story's "two-dimensional execution", arguing that its descent into a "waiting game" as the SKR4 draws closer to Earth makes the episode an "unfulfilling watch".