The Mars Probe 6 mission, led by General Carrington, made first contact with an alien race on Mars. It is implied that these aliens were not actually from Mars, or even possibly from our galaxy, but whatever their origin, they were already on the red planet when the Earth astronauts arrived. Jim Daniels, one of the astronauts, was killed after being touched by one of the aliens; Carrington understandably interpreted this as a hostile action, believing they wanted to invade our galaxy, and became determined to annihilate them.
Upon returning to Earth, Carrington became the head of the reformed Space Security Department. Somehow he found out how to get in touch with the aliens, and made an agreement with them to send three of their ambassadors to Earth. But rather than making an overture of friendship, Carrington intended to trap the ambassadors on Earth and arouse public opinion by alerting the world to the menace of an alien invasion. He believed their claims of being ambassadors were lies to put the Earth off its guard, and felt he had a moral duty to protect the world. His plan was to introduce one of the aliens at a worldwide televised press conference, and ask for the nations of the world to attack the aliens' spaceship with missiles equipped with nuclear warheads.
The aliens appear in two forms - over a videolink they appear to be fuzzy, like static, and blue colored, but in person, their flesh looks molten and bubbly. They communicate with high-frequency accelerated radio impulses, and their written language is pictographic (or, at least, they use a pictographic language to communicate with other species). Coded pictographic messages can also be transmitted in these radio signals. The aliens require a certain form of radiation to survive; Carrington hypothesizes that it is self-sustaining, highly contagious and could spread across the entire planet, but he is probably lying about all of this (it is, at the very least, not at all contagious, although it affects humans in the same way as terrestrial radiation). The radiation does, however, dissipate quickly.
The aliens have a touch that is deadly to humans. It is unclear what exactly their touch conveys - from the available evidence, it could be electricity, but it could equally well be some form of powerful radiation. One touch, even if the alien is wearing gloves, is instantly (although not always) fatal, delivering a bright red blast of energy. This power can be conducted through metal, and it is also intense enough to burn through metal if necessary. Apparently this power is generated all around their body; they are surrounded by a kind of forcefield that is strong enough to deflect and flatten bullets that are fired at them. The aliens are also very strong physically, and are not susceptible to G-forces.
They travel in a hollow orange discoid spacecraft which is half a mile wide in diameter and can travel through space in excess of 7000 miles per hour. These ships can emit radio pulses on frequences similar to that of pulsars, which seem to jam radio signals. Aboard these crafts, they can generate environments which are suitable for visitors such as humans, and condition their minds to keep them from deteriorating.
Carrington sends Mars Probe 7 to Mars, where the aliens intercept it and switch the astronauts with their ambassadors. When Earth sends a recovery shuttle to find Mars Probe 7, which has been lost for seven months, the ambassadors use it to travel down to the planet. After fulfilling their part of the contract and sending their ambassadors to Earth, the aliens send a signal containing instructions on building a machine which can be used to control the ambassadors. This machine sends impulses that they understand, and they will then do as they are directed. It is also possible to build machines that translate these radio impulses into human speech, and vice-versa. Unfortunately for the ambassadors, they are kidnapped by Carrington's men, who hold them prisoner and force them to kill, which they do not understand. The aliens do not want to kill, and had not realized on their first meeting that their touch could kill human beings; they came in peace. Meanwhile, the aliens remaining on the spacecraft threaten to destroy the Earth unless their ambassadors are returned to them. The Doctor agrees to help, and enlists the aid of the ambassadors to get back into the space center and stop Carrington before he can begin his broadcast to the world. He then arranges for the ambassadors to be returned to their spaceship, and the astronauts to be returned in exchange.
