Chief Scientist Styggron was a Kraal who devised a plan to invade Earth, allowing his species to leave their native Oseidon where radiation was reaching hazardous levels. Marshall Chedaki served him in this. For this scheme, he created the Kraal androids. Using a pattern analyser, the memory and intelligence of any living being could be fed into an android that then assumed the appearance of the subject.
Styggron brainwashed Guy Crayford, having him believe that the scientist had saved his life and that the Kraals wished to peacefully co-exist with the humans. On Oseidon, he created a replica of the English village Devesham and populated it with androids. This served as a training ground in preparation for the invasion of Earth and the conquests Styggron hoped would follow. He had Crayford, who had no android duplicate, take charge of the Space Defence Station, which was also copied.
When the Fourth Doctor and Sarah Jane Smith came to Oseidon, landing in the replicated Devesham, he ordered that they be taken prisoner, countermanding Crayford, who wanted them killed. He had Sarah analysed and a reproduction of her created. Styggron also built a duplicate of Crayford that would recognise Kraals as hostile. He destroyed it to display to Chedaki the power of a handheld weapon he had developed, which he hoped would be adapted for use on Kraal space cruisers.
The Doctor was taken prisoner and Styggron had him tied up in the centre of Devesham and a matter dissolving bomb planted by him. The bomb detonated, destroying the testing ground, but the Doctor was freed by Sarah before he was killed.
Styggron had developed a virus which would wipe out humanity in three weeks, allowing the Kraals to benefit fully from the Earth's limited resources. The Doctor and Sarah were recaptured with the Time Lord, at Crayford's insistence, being subjected to analysis. Styggron did so in the knowledge that the Doctor will be killed by the process. He also had food and water laced with the virus sent to Sarah, hoping to examine its effects upon her as previous tests had not been carried out on living subjects.
Styggron boarded Crayford's XK-5 space freighter which set off for Earth. The Doctor was on board, having been freed by Sarah who was with him. A number of the personnel at Devesham Control were already replaced by androids when the Doctor arrived, and a duplicate of the Doctor was introduced. The Doctor was able to use the radar dishes to disable the androids and free the real Harry Sullivan and Colonel Faraday.
Styggron confronted Sarah, questioning her on how she survived, but Crayford, who had discovered that he had been lied to, attacked the Kraal. Styggron shot and killed him but was then attacked by the android duplicate of the Doctor. It had been reprogrammed by the Time Lord and, in knocking him down, caused him to fall on the container in which the virus was being kept. As it killed him, he shot at the android, destroying it, but died without knowing that it wasn't the real Doctor. (TV: The Android Invasion)