At a zoo, the Doctor and the Brigadier are attacked by an escaped flock of parrots who have suddenly become inexplicable aggressive. The Doctor quickly traces the trouble to a group of visiting children from Arkwood Private School. As the Doctor and the Brigadier arrive at the school to investigate, ten-year old genius, Cedric Matthews uses his drug on his fellow pupils and leads them on a rampage of destruction.

The Doctor and the Brigadier attend a demonstration of new all-purpose tank-like vehicle called the multi-mobile. The vehicle is stolen by enemy agents who intend to use it to attack a nuclear defence centre. The army and the air force are unable to bring the multi-mobile to a halt leaving the way clear for the Doctor and UNIT to give their best shot.

A variety of giant insects suddenly start appearing in the countryside. The Doctor is able to track the cause down to the freak chemical reaction of a pesticide spray. As the creatures continue to terrorise the locals, the Doctor struggles to develop a formula that will reduce the insects to their normal size.

A meteorite crash lands on Earth where it is examined by the Doctor and Liz Shaw. While in transit, three groups of intelligent iron filings bore their way out of the meteor and cause metal structures to collapse, rusted away as they 'breathe' metal. Using an anti -corrosive fluid, the Doctor is able to neutralise the threat posed by two of the groups of filings. However, the Doctor is unaware of the existence of the third group which attach itself to a plane about to take off with a party of school children aboard.

After the Doctor and Liz assist the Brigadier and UNIT with the test of a new kind of depth charge, several ships in the same area are wrecked. The Doctor's suspicions are confirmed when he discovers the existence of an underwater race called Carpanthans. The Doctor and Liz are put on trial by the fishmen for the destruction of their city caused by the UNIT depth charge.

The Brigadier introduces the Doctor to Professor Logan who has masterminded a successful manned mission to Venus. Before he agrees to further funding for the Professor's project, the Brigadier asks the Doctor to examine some rock samples taken from the surface of Venus. However, the Doctor discovers that the rocks are in fact from Earth. The Doctor deduces that the Professor has faked the whole Venus mission in order to use UNIT's money to finance the upkeep of his castle, ancestral home of the Logan clan. Determined to keep his plan a secret, the Professor straps the Doctor into a rocket and launches it on a one-way trip to Venus.

Professor Carl Readon demonstrates his life's work to the Doctor - an intelligent robot with emotions. Soon after the demonstration, the robot apparently goes berserk, rampaging through the countryside. The Doctor Eventually realises the cause of the robots' actions is a small dog that it had become attached to but had been banished from the laboratory by professor Readon.

At the Request of the Home Office, the Doctor investigates a mysterious fire at remote country inn. The Doctor soon discovers that the culprit is a Professor Vinter who is using a group of Fire People who live beneath the Earth. Journeying to Africa, where the Fire People originate, the Doctor becomes trapped beneath a lava lake with Vinter and his flaming friends.

A millionaire called Henderson asks the Doctor to test out a newly invented time machine. Transported to the year 2971, the Doctor encounters a descendant of Henderson's who uses a slave labour work-force to do his bidding. The Doctor finds himself caught up in a war between the weaponless King Henderson and his neighbour, King Trent.

The TARDIS is directed by the Time Lords into Earth's future where the Doctor discovers that the world is now ruled by children with adults as their slaves. The children's leader, the genius Oswald, attempts to use his sophisticated electronic eye to interrogate the Doctor. When the Doctor resists the device and refuses to reveal the secret of the TARDIS, Oswald is forced to condemn him to death for fear of his own failure undermining his leadership.

The Doctor and Arnold attempt to warn a mysterious space fleet that is heading directly into a super nova. The steering mechanism of the TARDIS is damaged when one of the open fire on it. Along with the fleet, the TARDIS is drawn through the nova which is revealed to be an illusion. Landing on a planet in a hidden solar system, the Doctor and Arnold are captured by a race of spider-like creatures known as Spidrons. The Doctor learns that the Spidron emperor is the only intelligent member of his race and is sustained by living off primitive creatures abducted from other planets. Escaping from the Spidron's underground web-city to the surface, the Doctor and Arnold are confronted by dinosaur-like monsters and a race of primitive ape-like creatures. Somehow the Doctor must convince the apes that he and Arnold are their friends and that together the can defeat the Spidrons.

In space, the Doctor encounters another time/space craft which he guides back to its point of origin on nineteenth century Earth. The craft's pilot is the arrogant scientist Tobias Philby, who has incurred the wrath of the local villagers who believes his experiments are the work of the Devil. When the villagers set fire to Philby's castle, the Doctor saves the scientist and his manservant by escaping with them in the TARDIS. Landing on a battlefield in World War I, the three time travellers are captured by the Germans and accused of being British agents. After making a narrow escape from the firing ligne, the Doctor and his friends cross over into British territory only to be accused of being German spies. Somehow the Doctor must convince the British of the threat of approaching German forces and return Philby and his servant to their own time in the TARDIS.

The Doctor is called by the CID to investigates a mysterious bank raid where a huge vault door has apparently disappeared along with the money. The Doctor's investigations reveal the vault to be radioactive. The man responsible is Professor Pillbright, who is in the employ of a squad of Daleks hidden on the dark side of the moon. With a Dalek disintegrator weapon, the Professor intend to raid the Bank of England.

In Australia, Doctor French refuses to heed the Doctor's warnings that his new radio wave accelerator could spell disaster for Earth as well as for the alien cultures that he hopes to contact with it. With no option left to him, the Doctor resorts to sabotage but is caught before he can render the accelerator inoperable. The Doctor's fear are borne out when the accelerator is tested for the first time. The accelerated radio transmission causes disastrous disturbances in the weather on a global scale and subsequently lead to Doctor French's own death. The Doctor is more concerned about the fate of the planet that the transmission was directed at. Travelling to Morrax, the Doctor finds the world devastated and is taken prisoner by the surviving Morraxian people. The Morraxian leaders vow revenge on the people of Earth and dispatch a lethal missile with the Doctor trapped on board.

The TARDIS is taken aboard a giant space vessel where the Doctor discovers he has been taken prisoner by the Mantis, a race of giant insects. The Mantis are being pursued by their former masters, the Tyrryxians. Using their ability to control size, the Mantis shrink their ship down to the size of a microbe to avoid detection by the Tyrryxians. The Mantis leader is convinced that the Doctor and the TARDIS may enable him to escape the Tyrryxians but the miniature Doctor has escaped into the bowels of the ship. Determined to recapture the Doctor, the Mantis leader dispatches a beetle-like creature to retrieve his prisoner.

When the TARDIS lands in England's Middle Ages, the Doctor is captured by a villainous Lord Waldean de Beauvain and is imprisoned in his castle dungeon. The Doctor finds that he is sharing cell with Haval, once the Sergeant of Arms to the real master of the castle, Lord Geoffrey, who has been usurped by his brother. The Doctor devises a plan and the two escape but are soon imprisoned again, this time by the sorcerer Signus. The Doctor is able to revive the hypnotised Sir Geoffrey who they are imprisoned with. Constructing a glider, the Doctor enables Haval to fly from their tower prison and over the castle moat in a bid to seek help from the King's men. In the mean time, the Doctor and Sir Geoffrey must resist an attempt by Signus to take over their bodies by mystical means.

The Doctor is called to investigate the destruction of a television mast, apparently caused by metal-eating insects. The creatures have been breed by scientist Professor McTurk and his daughter Catriona. The two plans to use the insects to extort money out of British government in revenge for ignoring the Professor's talents. When the insects are released on an army firing range, their devastating attack on a tank battalion is witnessed by two foreign spies. The enemy agents trace the insects to the McTurk's farm when they shoot the Professor. Honouring McTurk's dying wish, catrional releases the insects to wreak havoc on the world. As the insects swarms toward Blackpool, the Doctor enlists Catriona's help to prevent them heading out to see and the rest of the world. One thing can save the Earth now - Blackpool Tower!

Making a successful moon landing, the crew of the Achilles discover a deep cylindrical shaft that has been tunnelled into the lunar surface. On Earth, the Doctor's warning against any exploration of the shaft are ignored. When Major Franklin disappears after descending down the tunnel, the mission is aborted. However, as the returning space capsule enters the Earth's atmosphere but it and the astronauts are turned to stone. The Doctor attributes the cause to fragment of the mineral Crunthel collected by the astronauts from the tunnel floor. Joined by CIA agent Harry Godino, the Doctor travels in the TARDIS to the moon. There they discover that the tunnel leads to the burial chamber of a member of an ancient alien race. Trapped in the tomb, the traveller's only hope of escape is for the Doctor to make contact with the aliens - the Lords of the Ether themselves.

Morun and Alto are the last surviving members of the crew of a giant space ship. In their charge is the whole of the Thusion race who are in suspended animation. Despairing of ever finding an uninhabited world to settle on, the dying Morun revives a young man, Zeros, from his long sleep. Zeros, however, does not share Morun and Alto's qualms about subjugating the primitive race of a nearby world. When the Doctor arrives, he saves Alto from death at Zeros's hands and attempts to use the TARDIS the whole Thusion ship to another more hospitable galaxy.

In space aliens plan the invasion of Earth but first Doctor Who must be eliminated. To this end, the aliens dispatch a small craft containing an insect like creature. Encountering the monster whilst walking in a park, the Doctor has only his walking cane to defend himself with.

At the Brigadier's request, the Doctor agrees to infiltrate a foreign embassy to recover an important device. Disguised as an old woman and using his obedience spray, the Doctor manages to gain access to the embassy and locate the device. However, as he does so, his presence is detected and the alarm is given.

A desert island castaway is using high-pitched sound to make a giant squid attack passing ships. The Doctor and the Brigadier are holidaying on the Brigadier's private yacht when it is attacked by the squid. Taken as prisoners, The Doctor and the Brigadier are imprisoned by the castaway in the cave.

While the Doctor teaches himself to levitate by the power of his mind alone, the Brigadier gives chase to an escaped spy. With his new-found ability to defy gravity, the Doctor attempts to help the Brigadier.

On Zenos, the natives are frozen by a petrifying ray emitted from an invading Groob ship. The Groob leader, Parada is intent on ransacking the planet for its mineral wealth. Sent by the Time Lords, the Doctor lands on Zenos and is captured by the Groob. By tempting Parada with the ability to travel in time, the Doctor hopes to be able to free the people of Zenos and rid them of the Groob menace.