Published in the TV Century 21 magazine #44-51, November 20, 1965 - January 8, 1966.
Receiving in the post a strange flashlight which emits an invisibility beam, Lady Penelope has no idea who sent it to her or why and that presents a mystery she so enjoys. Unfortunately for her and Parker, a couple of others want that torch.
Note: this story also shows how Lady Penelope would go to work for International Rescue.