one of our spies is missing (1966)

Unknown.
UNCLE agent Illya Kuryakin buys himself the 'kind of cat that gets itself stolen', and releases it in the street, and waits for someone to pick it up. It turns out cats are being stolen to use in experiments to reverse the ageing process.

Unknown.
This is revealed when Illya finds a house full of caged cats, some of whom have become kittens, 'you were a big cat last time I saw you?'
Unknown.
Meanwhile Napoleon Solo visits a rival agent, who also has a cat. She slips a mickie into the cream for his coffee, but he discovers this when he offers some to the cat, which makes it plain it doesn't like it!