The one where the Master teams up with the Doctor to defeat the terror of the Autons...
In the past three episodes alone, the Master has tried to kill the Doctor at least four times - first by rigging a volatiser trap in the radio telescope, then by hiding a bomb inside a UNIT crate, then by trying to blow him up with a grenade held by Phillips, and then by using a sentient trimphone cord. So when the Master suddenly appears in the Doctor's lab (what was he doing up the spiral staircase?), pointing a weapon right at him, you expect him to be able to see his threat through.
But oh no, like so many arch-villains in adventure fiction, he has too much to say for himself and ends up missing the easiest opportunity for killing the Doctor he's yet had. All he had to do is point and shoot, but his urgency to get back to the bus at the quarry means he loses all reason and instead orders the Doctor to drive him there in Bessie! I find it hard to believe the Master doesn't really mean to kill the Doctor, because he's tried in four pretty uncompromising ways already. If this is all just a game to the two of them (as has been suggested in some quarters of fandom), then the Master doesn't mean the game to last very long!