A cargo shuttle arrives on the planet Inter Minor where a local tribunal, Kalik (Michael Wisher), Orum (Terence Lodge) and Pletrac (Peter Halliday), receive two Lurman visitors, the showman Vorg (Leslie Dwyer) and his assistant Shirna (Cheryl Hall). They have brought a Miniscope with them. 

The TARDIS arrives in the hold of a ship. The Doctor is puzzled as he was heading for Metebelis 3, the blue planet of the Acteon galaxy. The Doctor and Jo explore the ship and hide when Major Daly (Tenniel Evans), his daughter Clare (Jenny McCraken) and Lieutenant John Andrews (Ian Marter), a seaman, enter the saloon. Clare and Andrews leave to walk around the deck and Daly falls asleep. Jo sees a copy of the Illustrated London News on the table. It is dated 3 April 1926. Suddenly a plesiosaur rises from the sea. In the confusion, the Doctor and Jo try to leave but are seen by Daly. Andrews says they are stowaways and locks them in a cabin. There, the Doctor discovers that the ship is the SS Bernice and the date is the 4 June 1926 - the day that, according to history, the ship vanished. Jo notices that when they entered the clock read 7:35 and now it reads 6:40 and it is also daylight outside when it should be night. Just outside the cabin door is a hexagonal metallic hatch in the floor that Andrews cannot see and, having used Jo's skeleton keys to escape from the cabin, Jo and the Doctor examine it. 

On Inter Minor, the tribunal decide to reject the Lurmans' visa application. Vorg has a document micrographed by the Great Zarb and Pletrac takes it away for verification - Shirna realises that 'the Great Zarb' is a Wallarian wrestler and not the President of Inter Minor. 

The Doctor determines that the hatch is secured by anti-magnetic cohesion and that he needs a magnetic core extractor to open it. He has one in the TARDIS and so he and Jo head for the saloon. There, they see Daly and the others going through exactly the same sequence of events as before. They return to the hold and the Doctor fetches the piece of equipment from the TARDIS before a giant hand suddenly emerges from the ceiling and takes the TARDIS away. 

Vorg has found the TARDIS in Circuit Three. He replaces it inside the Scope's field as Kalik and Orum ask what the Scope does. Vorg explains that it contains miniaturised life forms from across the galaxy, including Tellurians, an Ogron (Rick Lester) and Drashigs. To demonstrate that the pictures shown on the glo-sphere are not recordings, Vorg turns up the aggrometer which makes the specimens behave more aggressively. 

On the ship, the Doctor and Jo have again been spotted by Daly, but this time Andrews challenges the Doctor to fisticuffs. The Doctor wins the fight and he and Jo hurry to the hatch chased by armed sailors. Jo starts to unseal it but Andrews and the sailors arrive. Vorg turns the aggrometer off and Andrews and the rest of the crew return to their duties without a second glance at the Doctor and Jo. Opening the hatch, the Doctor and Jo descend into the workings of a giant machine. 

Pletrac says that the machine must be destroyed as it contains livestock. He orders some functionaries to bring and operate an eradicator which fails significantly to affect the scope or its contents, which include a Cyberman (Terence Denville). Vorg sees the Doctor and Jo in the machine. Orum removes the TARDIS from the Scope and it grows to its normal size. 

Trying to find a way out, the Doctor cuts through a restraining bar over another hatch and he and Jo find themselves in a cave. Outside the cave is a marsh inhabited by ravening caterpillar-like Drashigs which chase the Doctor and Jo. Shirna gets Vorg to distract the creatures allowing the Doctor and Jo to return to the machine. The Drashigs - which hunt by smell - follow. The Doctor realises that they are inside a Miniscope and tells Jo that he had been instrumental in getting the High Council of Time Lords to ban them. 

Kalik has seen an opportunity to overthrow Zarb and to gain power for himself by using the Drashigs. He and Orum plot against Zarb. 

As the Drashigs roam through the machine's innards, the Doctor and Jo return to the ship to get some rope with which to reach the base of the Scope. A Drashig smashes through into the ship and is killed by Daly with a machine gun. Andrews throws some dynamite into the machine to try and kill another creature. He succeeds, but causes a general power failure in the scope. 

Jo is held on the ship, but the Doctor manages to escape from the Scope and grows to his full size. 

Kalik has disabled the eradicator by removing its tryizon and so prevents Pletrac from using it on the Doctor. The Doctor takes control of the situation. He wants to rescue Jo and return the rest of the 'livestock' to where it originated. Kalik places the component from the eradicator in Vorg's luggage so that the Lurman will be implicated. A plate at the base of the Scope is moving as the Drashigs try to break out. The Doctor intends to link the machine to the TARDIS and then re-program it. He builds a device to achieve this and instructs Vorg on how to use it. He is then transported back into the Scope to find Jo. 

While he is gone, Pletrac destroys the device, leaving Vorg to try and repair it. Vorg finds the eradicator component in his bags and wonders how it came to be there. In the scope, the power runs down and the life support systems fail. 

Kalik releases the Drashigs from the machine and becomes their first victim. Vorg fits the tryizon to the eradicator and destroys the Drashigs before they can kill anyone else. He then operates the repaired machine which rescues the Doctor and Jo and returns the other life forms to their natural habitats. The scope then blows up. 

Vorg is a hero and to earn some credit bars, he demonstrates the yarrow seed and megum pod trick to Pletrac. The Doctor and Jo leave in the TARDIS.

_{Synopsis from Doctor Who: The Third Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos}