The first time the fearsome Kraals attempted to take over the Earth, with the help of their android agents, things didn't go quite according to plan — thanks to the time-travelling alien Doctor and his allies at UNIT.
This time, Marshal Grinmal and his belligerent cohorts are ready for them. This time, they'll make no mistakes. This time, Chief Scientist Tyngworg has not just one plan, but a back-up plan, and a back-up back-up plan worked out...
With the Doctor a prisoner on the Kraals' radiation-blasted home planet of Oseidon, only his companion Leela can save the day — alongside a most unlikely ally.
The Master has allied himself with the Kraals, who invade Earth through the space-time wormhole created by the Great White Worm which is linked to their home planet Oseidon. Marshal Grinmal is the commander of the invasion force which it composed of the second Kraal army.
Leela describes the Kraals as resembling "a pig who has mated with a rhino." The Doctor tells her that twelve centuries of living on a planet which has been ravaged by nuclear radiation will not result in a species which is "easy on the eye."
Marshal Grinmal forges an alliance with Colonel Spindleton, who agrees to offer any and all assistance to the Kraal invasion force on the condition that he be allowed to rule England once the Kraals have conquered.
Marshal Grinmal has the Master chained up in Colonel Spindleton's stables. Leela rescues him as she requires him to pilot the TARDIS so she can retrieve him from Oseidon.
After his arrival on Oseidon, the Kraal Chief Scientist Tyngworg continues the transposition of all the Doctor's knowledge and experience into a databank in the Disorientation Centre, a process which his predecessor Styggron had begun years earlier. Tyngworg creates an android duplicate of the Doctor, who does not realise that he is not the genuine article. It is the android that Leela rescues from the Disorientation Centre.
It is revealed that Tyngworg is in fact the Master in disguise while the version of the Master which the Doctor and Leela met in Dark Peak and who travelled with the latter to Oseidon was an android duplicate. The Master destroys him as he has outlived his usefulness.
As is still connected to Tyngworg's analyser, the Doctor programmes his own android duplicate to create an android duplicate of Tyngworg who turns the androids against the Kraals.
The Master who had disguised himself as Tyngworg is revealed to be another android duplicate. The Doctor determines that the entire Kraal invasion was intended as a distraction which would allow the real Master to carry out his plans undisturbed.
On Oseidon, the Doctor and Leela create a third android duplicate of the Master, which likewise believes himself to be the actual Master.
Using Colonel Spindleton's forces, the Master captures UNIT HQ in order to gain access to the Doctor's former laboratory.
Prink's Theorem states that ZO-radiation is formed when Z-radiation and O-Radiation, both of which are harmless on their own, meet. ZO radiation is able to emit the same level of energy as a billion neutron stars. The Master intends to absorb this energy, which will not only provide him with a new regenerative cycle but make him effectively immortal.
The Master foresaw that one of his android duplicates would learn his plan and set an explosive device to detonate in 30 seconds if the deleted files concerning the ZO radiation were accessed.
During his third incarnation's exile on Earth, the Doctor attempted to use a Z-battery to jump start his TARDIS. His attempt failed and, after his exile ended, he left the Z-battery behind him in his lab.
The Brigadier is still working for UNIT, and still remembers the Doctor. Therefore, it may be prior to or during 1976 rather than 1979 as he has not retired yet and has not lost his memory of the Doctor. However, given the UNIT dating controversy, it is far from clear. (TV: Mawdryn Undead)