In this episode of Poirot, "Hickory Dickory Dock," from season 6, Poirot offers to help Miss Lemon's sister, who runs a hostel. Objects are missing from the rooms, and some of them are strange: a shoe, a belt, a bracelet, a stethoscope - almost sounds like a scavenger hunt.
One of the women living at the hostel, Celia, announces that she is a kleptomaniac. Poirot is told by a fellow student that it isn't true -- she was trying to attract a psychiatry major, so she stole things to be more interesting to him. Celia also says that she didn't steal everything but knows who the other culprit is. She then dies when someone puts morphine in place of a sleeping powder. That's the first murder. More to follow.
It's an intriguing case which underlies a much bigger one that includes smuggling, hidden identities (several), and the murder of a statesman's wife ten years ago that has similarities to the current murders. Japp always held the statesman responsible for his wife's death; now he is dying in the hospital.the thief to the grave.
Meanwhile, Japp's wife is out of town and he's a mess, so Poirot invites him to move in until his wife returns. Japp is then subjected to Poirot's idea of food and is mystified by the bidet. Very funny.
Some of this mystery isn't hard to figure out if you've seen hundreds of this type of thing as I have, but it's still a good mystery with several layers. This isn't really an early episode, as it's from season 6, but it retains some of the first Poirots in its humor and the personality of Poirot. And that makes it fun to watch.