Board Thread:The Panopticon/@comment-4028641-20151008023030/@comment-6032121-20191111113723

There's two things.

First Susan flays his mind with her psychic powers, reducing it to the ball of concentrated hatred (devoid of Delgado's silky charisma) portrayed in The Deadly Assassin, and blows him out of his TARDIS and onto Tersurus.

Then, in a separate scene, as they stand together on Tersurus, Susan asks the Master to surrender the Dalek weapon he's stolen, lest she be forced to destroy it with him still clinging to it. In his diminished state of mind, the Master refuses to let go, so Susan shoots the device while the Master's still holding it, causing an out-of-control explosion that warps and burns the Master's flesh into Crispy form in a way from which regeneration cannot save him.

Then, after Susan leaves aboard his TARDIS, he is found by Goth in precisely the state in which he begins The Deadly Assassin in yet a separate scene.