Still weak and confused after his fourth regeneration, the Doctor retreats to Castrovalva to recuperate. But Castrovalva is not the haven of peace and tranquility the Doctor and his companions are seeking. Far from being able to rest quietly, the unsuspecting time travellers are once again caught up in the machinations of the Master. Only an act of supreme self-sacrifice will enable them to escape the maniacal lunacy of the renegade Time Lord.
The Fifth Doctor discovers that his attempt to return Tegan to Heathrow Airport has brought the Tardis to a spacecraft that appears abandoned at first, but actually carries a force of frog-like aliens that may challenge the future of humankind on Earth.
The Tardis visits the planet Deva Loka, where Nyssa remains behind in the ship to recover from a mild mental disorientation while the Doctor, Tegan and Adric explore. Tegan falls asleep under some wind chimes and becomes possessed by an evil force, a Mara.
The Doctor attempts to take Tegan back to Heathrow Airport but the TARDIS arrives in the 17th Century instead of the 20th. The time travellers discover that a space capsule has crash-landed nearby and that its alien occupants, three Terileptil prison escapees, intend to wipe out all indigenous life on Earth by releasing rats infected with an enhanced strain of the great plague. The creatures are also using a sophisticated android to strike terror into the local villagers. Aided by itinerant thespian Richard Mace, the Doctor tracks the Terileptils to their base in Pudding Lane, London. The creatures are ultimately destroyed when a fire breaks out and the Terileptil leader’s weapon explodes – also setting off the Great Fire of London.
The TARDIS arrives on Earth in 1925 where, due to a case of mistaken identity, the Doctor ends up playing in a local cricket match. The travellers then accept an invitation to a masked fancy dress ball, but events take on a more sinister tone as a number of murders are perpetrated at the country home of their host Lord Cranleigh.
A fateful landing on Earth in the 26th Century finds the TARDIS deep within a system of subterranean caves and even deeper within the mystery surrounding the brutal murder of a group of geologists working there. Who is controlling the androids guarding the hatch in the rock face? More importantly, what lies beyond the hatch?
The answers provide a serious threat for the Doctor as the time and place is set for an encounter with his old arch enemies. The Cybermen are well and truly back and the Doctor and his companions could well be counting the cost in truly personal terms.
The Doctor finally manages to deliver Tegan to Heathrow Airport, where he gets drawn into investigating the in-flight disappearance of a Concorde. Following the same flight path in another Concorde, with the TARDIS stowed in the hold, he discovers that it has been transported back millions of years into the past through a time corridor.