The Lords of
the Red Planet

The journey itself is eventful, but when the Doctor, Jamie and Zoë arrive on the Moon, it is to find that it has been taken over by the Ice Warriors, under the supervision of two Martian Warlords -- a more intelligent and humanoid version of the Warriors.

The Martian plan is elaborate. By taking over the despatch centres of T-MAT, they have immediate access to the major populations centres of Earth -- and similarly if they require, all the subsidiary planets in the system. They intend to send one Warrior to each of these cities -- not to attack or destroy, but to plant the spore-bearing algae that normally grows on Mars. This plant is adaptable, but basically anaerobic -- doesn't need oxygen. Its spores contain an hereditary virus which breaks down the chlorophyll molecule.

With chlorophyll destroyed the process of photosynthesis in plants is impossible -- thus, no oxygen is produced to replenish Earth's natural supply, the carbon dioxide level of the atmosphere increases, and the "free" nitrogen content of the air becomes unbearable. This in addition to the toxic gas produced by the spores at close range, will soon make Earth uninhabitable to humans -- but perfect for the coming of the Martians. The main body of these are waiting in deep space, for the peaceful invasion that will take place when Earth is "prepared" for them. The spores "burst" and spread inside four weeks -- and once the secondary coverage is achieved there is no hope for human life.

Having discovered the essential purpose of the "suicide Warriors", the Doctor's plan is twofold -- first to destroy the spores before they burst, and then to somehow beat back the Warriors -- who, if they cannot conquer by their more subtle method, will invariably use force, even though this will mean having to repair the destruction.

The spores are eventually destroyed by being sprayed with oxygen -- possibly liquid, for visual effect -- while the Warriors are allowed to enter the despatch chambers, but diverted into open space, whilst still in their dematerialised state. The Professor is left happily in charge of an emergency transport system

In November 1968, it became apparent the Hayles' scripts were not what the production office wanted. Therefore agreement was reached with Hayles that assistant script editor Terrance Dicks could rework the final four episodes, for which he would share the writer's credit (although not on screen or in publicity material).