A boy lives in the French embassy in London. He takes a brick out of a wall on an upstairs balcony and behind it there is a legless lizard that looks like a Slow Worm, which is the only limbless lizard found in England. He holds it and kisses it and talks baby talk to it, calling it McGregor (it looks dead) then we cut away to him putting it on the ground where it is now a completely different reptile - a small snake with a long thin tail, probably a Grass Snake. It looks like the lizard was chilled down so much that it wouldn't crawl, so they substituded a snake, or the snake was too squirmy to use when the boy was holding his pet. When he picks it up again, it's the lizard. After that every other time it is shown it is the lizard. He puts it in his shorts pocket. The angry housekeeper asks if he put it in the lavatory as she asked him to do. He takes it to the kitchen, puts some food in water and tries to feed it. It is now moving around. He carries it to a cafe. Then puts it back under the brick. The housekeeper sees him, and when he leaves she goes out to the balcony. We later see her coming downstairs with something wrapped up in paper, which she throws into a stove and burns. It's the lizard. When he finds it missing the next day he cries.
This is a legless lizard, but I'm not sure if we're supposed to think it's a lizard or a snake. I doubt many who watch the film know that there are lizards with no legs. I read somewhere that Graham Greene, wrote the short story "The Basement Room" that the film is based on and the
screenplay, liked snakes, so that makes me wonder if the filmmaker intended the animal to be a snake - thinking nobody can tell the difference. I don't remember if anybody ever called it a lizard. The boy had no pet in the short story on which the film was based, but Graham Greene was also the screenwriter.