The Doctor, Jamie and Zoe find a cat-like creature when the TARDIS lands on a strange planet. It is well-cared for but they can't find its owner; the Doctor decides to take it with them. They name the "cat" Marmaduke because he is marmalade-coloured.
Jamie is amused when Marmaduke becomes lodged in the temporal relay circuits and the Doctor has to get him out. It wasn't the first time that Marmaduke had caused trouble. He shredded much of the clothing in the TARDIS wardrobe; he clawed holes in Jamie's bagpipes; he played with the Doctor's recorder and scratched the Doctor when he reached for it; he sharpened his claws on the TARDIS walls.
Then the Doctor starts behaving strangely. He answers Jamie and Zoe's remarks or questions before they can say them out loud. Marmaduke is amplifying his telepathic abilities. When the travellers return from an escapade one day, they find the control room in a mess. The Doctor decides they must get rid of the cat.
They try to return to the world from which they took Marmaduke, but it is gone. Instead they travel to another planet and leave Marmaduke in a basket on a porch. When they sneak back into the TARDIS, Marmaduke is waiting for them inside.
They try many ways of getting rid of Marmaduke: putting him on a train, leaving him on a hyperspace cruise liner, depositing him with a rescue centre, and leaving him with the Sisterhood. Each time, when they return to the TARDIS, Marmaduke is waiting for them.
The Doctor finally figures out how to get Marmaduke back where he belongs. They land back on the planet and encounter an old woman. She calls Marmaduke an Id-cat. She is a telepath who sought solitude to escape from all the other thoughts in her mind, but because she was lonely, she invented Marmaduke. He amplifies her powers, so the Doctor has brought him back centuries after they first picked him up — when the old woman can no longer mask her presence as she did before.
The Doctor returns to the TARDIS, wondering if he has just defeated a menace — or lost a companion.
- Marmaduke destroys Jamie's bagpipes and plays with the Doctor's recorder. He sits on Zoe's computer keyboard.
- The stories in Short Trips: Zodiac are inspired by the signs of the zodiac. This story's theme is Leo.
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