Doctor Who: 9 Ways To Kill The Doctor Permanently

The Master is essentially a sinister mirror of the Doctor, so it's quite fitting that the character's tool of choice reflects this. Instead of carrying a sonic screwdriver - which the Doctor normally uses for harmless purposes like breaking locks and scanning life forms - the Master carries a laser screwdriver, a weapon that is used for killing.

The Master started wielding this device in the modern era of the show, with the John Simm incarnation using it on several occasions throughout his tenure, and in his most recent appearance - Series 10 finale The Doctor Falls - it's shown that the laser screwdriver can kill Time Lords and inhibit their regenerative abilities.

We see this when the Master uses it to kill Missy. He zaps her in the back before informing her that she shouldn't bother trying to regenerate, because the full blast of the screwdriver was enough to kill her outright. With this in mind then, we can also assume that the screwdriver would have the same deadly effect on the Doctor.

Sure, Missy did eventually regenerate into a fresh body and went on to battle the Thirteenth Doctor, but as is customary with the Master, Missy's seemingly miraculous survival was never explained in the show. The only thing we're explicitly told is that a full blast of the laser screwdriver means no regenerations, so realistically, the Master's continued existence is just a "because reasons" kind of thing.