Dr Tyler responds to a call from Mr Ollis: Tyler's experiment has landed in Ollis' bird sanctuary. The disappearance of Ollis and other curious events convince Tyler to contact UNIT. Prints he had developed previously show a strange phenomenon, directed at Earth, which appears to be travelling faster than light. Using the Doctor's equipment, Tyler develops his latest print, while the Doctor and Jo go to the sanctuary to investigate Mr Ollis' disappearance. While they are gone, Tyler checks his instrument package and something emerges. Tyler disappears and the strange substance slides down the laboratory sink and into the drain.

When the Doctor and Jo return, they are attacked by the same substance as it emerges from a drain near the garage. Luckily they escape but it is not long before other creatures appear and begin to attack UNIT. Forced to take refuge in the TARDIS, the Doctor is faced with the fact that the only people who can help him now are the Time Lords. He sends out a distress signal, little knowing that the Time Lords are also under attack. A mysterious beam is draining their energy reserves. They had hoped to ask the Doctor help - it now appears that the Doctor must help himself, literally...

Vorg and Shirna are travelling showpeople: they arrive on the planet of Inter Minor via a cargo thruster. Not exactly first class but it gets them to their next showground. Inter Minor is emerging from a protracted period of isolation and there is still a distrust of aliens. A tribunal of officials, comprising Kalik, Orum and Chairman Pletrac, has been set up to consider whether or not to allow Vorg and Shirna to remain or if they should be deported. For a start they need to understand entertainment: but Vorg's miracle of intergalactic technology - the 'scope, his carnival of monsters showing monsters in their little habitat warrants greater attention. Initially, the tribunal assume this carnival of monsters is a series of recorded images. Vorg is horrified, the 'scope is good old fashioned live entertainment.

The Doctor and Jo emerge from the TARDIS into an environment that definitely does not match the description of Metebelis 3, the fabulous blue planet of the Acteon Galaxy. As they investigate their surroundings Jo concludes that they are in the hold of a ship sailing an ocean on Earth. The Doctor is convinced that they are on some planet within the same system as Metebelis 3 - but definitely not Earth! As they roam the ship the evidence seems to supports Jo's theory: the crew and passengers are quite human; it is June 4th 1926; they are on a cargo ship 2 days out of Bombay; sailing on the Indian Ocean. The Doctor wants to leave, there is something wrong - Jo wants him to admit he has simply travelled back in time rather than land on some other world. The appearance of a plesiosaurus in the nearby waters shocks both the passengers and crew of the SS Bernice and the Doctor and Jo. When they try to get back to the TARDIS they are captured by the crew and locked up as stowaways. En route, they pass a metal plate in the deck - none of the crew of the ship can see it. The Doctor and Jo are faced with a puzzle: it should be dark, but its broad daylight outside; the metal plate which seems to be invisible to the crew; the plesiosaurus; the Doctor's recollection that the SS Bernice vanished without a trace on June 4th 1926; and the fact that the clock in the cabin now reads an earlier time...

The pilots of an Earth cargo ship discuss the possibility that Earth and Draconia might plunge into war with one another. There are stories of each side carrying out raids against the other. The pilots' main priority is to make the jump to hyperspace - routine enough, until they nearly collide with some kind of object. Pulling back into normal space the pilots avoid the collision. However, another ship arrives - and it appears to be a Draconian ship - the stories of raids appear to be true.

The TARDIS has materialised in the hold of the cargo ship, the Doctor's own way of avoiding the collision. While he checks the instruments, Jo spots another ship close to the ship they are inside of. Both the crew of the ship and the Doctor and Jo are caught up in a skirmish. When another Earth ship arrives, the attacking ship has long gone. The pilots of the cargo ship begin with saying it was an attack by the Draconians, but the Doctor and Jo insist that the attackers were Ogrons! Accused of being agents working for the Draconians, the Doctor and Jo are imprisoned and taken to Earth to face trial. Matters are not helped when the Draconians apparently stage an attempt to free the Doctor and Jo. Again, the travellers see Ogrons while everyone else sees Draconians. When the Doctor meets a real Draconian he is accused of being an Earth agent trying to set up the Draconians. What do the Ogrons hope to gain by stirring up the peoples of Earth and Draconia? With the Doctor facing life imprisonment on a penal colony on the Moon and the TARDIS gone, taken by the Ogrons, it looks as if they may be caught up in an interstellar war!

The Time Lords have sent the TARDIS to another planet at the Doctor's request. Seeing that the Doctor needs medical attention, Jo leaves the ship to find help. Finding herself in a jungle filled with hostile plant life she finally locates a ship. There she meets a small group of people. At first they are suspicious of her but as she explains about herself and the Doctor, the group become interested. They agree to try and help him. Jo is told to remain hidden in the ship as there is a patrol nearby. They locate the TARDIS and free the doors from a substance growing over them, allowng the now recovered Doctor to get out.

The Doctor recognises the three men as Thals, natives from the planet Skaro - homeworld of the Daleks. He had asked the Time Lords to send him to this planet in search of the Daleks and he is horrified that Jo has wandered off on such a planet. Even more alarming is the discovery that the Daleks have come to this planet, Spiridon, in search of the secret of invisibility.

A protest outside of Global Chemicals, by miners fearing that their jobs are under threat and a small group from the Wholeweal Community, led by Professor Jones, is interrupted by the whistle from the nearby colliery. A miner is found slumped by the switch, dead, but his body is glowing bright green. Jo, already keen to get involved in world events, turns down another offer from the Doctor to attempt to visit Metebelis 3 in favour of going to Llanfairfach to help Professor Jones and his community stand up to Global Chemicals. The Brigadier is on his way to Llanfairfach: Global Chemicals is a vital industry and there are fears of sabotage so UNIT has been asked to help. Not even the matter of the miner found glowing bright green is enough to dissuade the Doctor from going to Metebelis 3. The Brigadier offers Jo a lift to Llanfairfach as they are both going there, albeit for different reasons, while the Doctor takes the TARDIS to Metebelis Three.

Jo meets Professor Jones and starts off on the wrong foot, nearly ruining one of his experiments. The Brigadier explains to Stevens, the head of Global Chemicals that he is there to help them but also to investigate this mysterious death. The Doctor's trip to Metebelis 3 proves to be more than he had anticipated and he returns to Earth in time to agree to come to Llanfairfach and help the Brigadier. When another miner requests help, Jo offers her assistance and descends into the pit with a miner, Bert. However, a fault with the lift mechanism strands them down in the mine...and the Green Death is spreading.