Doctor in the House

A Man's Best Friend Is His Cat (5x11)

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Sir Geoffrey and his wife are visiting her mother and he asks Duncan to look after his cat, Thomas, which wees everywhere and is not good-natured. Thomas gets put with the lab animals and Duncan assumes he has been taken away for experimentation. He buys a replacement cat but Sir Geoffrey claims this is not his Thomas. Thomas is saved, making Sir Geoffrey think again about animal tests, but, oddly, gives birth to kittens. Lady Loftus explains that the first Thomas died and she replaced him behind her husband's back, unaware that this cat was a different sex.