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. 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.