Two children playing hide and seek use the TARDIS as a hiding place and, while exploring, accidentally become stowaways on the ship.
Amy Barker and her brother, Tony, are playing hide and seek when Amy and their dog, Butch, use the police telephone box as their hiding place. Amy believes that the fantastic box is a some sort of spaceship and begins to explore. Tony is warier, but eventually follows his sister. When his elbow hits the door, it swings shut and the children are locked into the TARDIS.
Soon after the First Doctor returns, unhappy that he had landed on Earth in 1966 instead of some faraway world. He dematerialises the TARDIS and leaves for another world and time. When the TARDIS materialises, the Doctor realises that the TARDIS is not on firm ground and he grabs a flashlight to go out into the dark to investigate. After the door opens, the TARDIS lurches and throws the Doctor out. After a short fall, the Doctor switches on his light and sees that the TARDIS has landed upon a giant web which also snagged him, saving him from a great fall. The Doctor surmises from the darkness and the web he is in a great cavern. When he looks toward the TARDIS, the lights shining through the open door illuminates a giant spider, considerably larger than a full grown man. The Doctor finds that the spider cowers when the light from his flashlight is shined upon him. He uses his new found knowledge to make his way from strand to strand and back to the TARDIS.
The Doctor prepares a syringe of prussic acid, and when he sprays it at the spider, both it and the TARDIS fall to the floor of the cavern. As he again steps out of the TARDIS, the Doctor notices a perfectly round hole covered by a door. When the door swings open, bright light fills the cavern and the Doctor climbs through.
The Doctor finds himself in a cage, viewed by humanoid creatures with grey skin and no clothing known as Sensorites. When the Doctor demands to be released the Sensorites shrink back, fearful of him because he came out of the spider's lair alive. While some wish to destroy him, others are intrigued and reach through the bars to touch him, and he places the syringe safely out of reach. He soon finds that the spiders, known as Zilgans, are used by the Sensorites to punish their criminals. They take the Doctor prisoner for killing the Zilgan.
Tony and Amy wake under a bench where they hid when they found they were locked in the TARDIS. Tony remembers seeing an old man working controls before falling asleep, but doesn't know where he has gone. Seeing the door open, they prepare to venture outside just as the ship begins its fall to the cavern floor. Having been shaken during the fall, the children finally exit the TARDIS and see the porthole and giant spider. Tony realises that they must not be on Earth anymore. Exiting through the porthole Tony sees the planet's two suns. Amy and Butch soon join him and they set off to explore.
The children chance upon a large amphitheatre and see the old man strapped to a circular dais in the middle. Butch scampers to the dais and begins licking the Doctor. Seeing the dog and children, the Doctor at first believes he is hallucinating until Tony explains how they had accidentally travelled with him.
The Doctor and the children plan an escape. Knowing the Sensorites fear the dark and cannot handle loud noises, he has the children enter a small underground passage they found and tells them to begin making loud noises as soon as he is unbound.
As the Doctor waits, he begins to grow drier from his exposure to the dual suns. After what seems like hours, the Sensorites finally return and the Doctor finds that he is condemned to death for the killing of a sacred Zilgan. However, instead of being sent back into the cavern to be eaten, the Sensorites are determined to find a different method of destroying him.
As the bonds are removed so the Doctor can be transported to his execution, he lets out his loudest yell and the children and their dog join in. The Sensorites, fleeing the noise, allow the children to come to the Doctor's aid and assist him in escaping.
The three make their way back to the porthole and return to the cavern and the TARDIS.
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- The Doctor is referred to throughout the story as "Dr. Who". At one point, he identifies himself as "an intelligent human being".
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