A planet called Kraal has become dangerously radioactive. And this race,lead by surgeon Styggran,are planning to invade Earth with a group of androids carrying a deadly virus that will destroy humanity and leave the planet open to Krall colonization. Upon using their mind/body replication device to replicate the doctor,Sarah and the genuine doctor escape back to Earth on the Kraal rocket intended to deliver to deadly androids. Meanwhile confusion runs so amok at the real UNIT HQ at the empty TARDIS's appearance they are not aware of the true intentions of Crawford's return. Not to mention that Harry Sullivan and Benton have already been replaced by androids-as well as the doctor himself,and are already laying the groundwork for the proposed invasion. Upon successfully averting the invasion,the real doctor also stops a coup by the Androids when Crawford switches sides and is killed by Styggran,the doctor and Sarah return to their journey in the TARDIS.
As a whole this story is fully representative of all of the best qualities of Doctor Who during the mid 1970's. The chemistry between Tom Baker's fourth doctor and Elizabeth Sladen's Sarah-Jane Smith serves as the main strength motivating this entire adventure. And it's also a very compelling story at that. It mashes up both The Andromeda Strain and Invasion Of The Body Snatchers in order to comment on the pretentiousness of Earth defense forces to successfully stop an intricately planned extra terrestrial invasion. With it's very clever use of what minute special effects Doctor Who was given at the time in terms of available budget,this story managed to visually convey the sense of uncertainty and deception it was looking to convey. And again as was typical for the era,the fact that the doctor was in a type of danger that genuinely effect his life and that of his beloved Earth added to the strengths of this story as well.