Anti-thief device

The anti-theif device was a protective mechanism the Third Doctor incorporated into Bessie to keep her from being stolen. It was activated by flipping a small switch on the car's dashboard next to a red placard entitled "ANTI THIEF DEVICE". This would generate a powerful force field around Bessie that pinned any potential carjackers to the vehicle and kept them from breaking free until the device shut itself off, presumably to leave the would-be thief trapped and eventually apprehended. It could also be used to temporarily incapacitate individuals as a means of purely detaining them.

The Doctor was first seen using the anti-theif device against a group of criminals who had stolen a large freighter carrying the Recovery 7 landing capsule. The Doctor posed as an elderly bystander with his car broken down on the road, prompting the men in the truck cab to get out and help him push. This gave the Doctor a chance to set off the anti-theif device and pin them to Bessie, allowing him to reclaim the Recovery 7 capsule. When UNIT recovered Bessie, the Doctor noted they should have found two "very angry men" attached to the car, but Brigadier Lethbridge-Stewart claimed his men found none. The Doctor remarked that he needed to increase the power of the force field if the men were able to escape so quickly. TV: The Ambassadors of Death)