Although early in their onscreen rivalry, this episode provides some fascinating insight into the past relationship between The Doctor and The Master. The Keller Machine uses negative thoughts deep inside the mind of the victim and uses those thoughts to destroy the victim, then feasting on those same negative impulses.
Twice in this story, The Doctor has an almost deadly experience with the Keller Machine. During his first encounter, he is tormented by images of fire, a nice callback to the events of Inferno and showing that The Doctor's past adventures do indeed leave an effect on him. The Doctor's second meeting with the machine bridges Episodes Three and Four, but this time, The Doctor is haunted by images of old foes - Daleks, Cybermen, Ice Warriors, Zarbi (Zarbi?), and so on. After The Master manages to switch off the machine, he seems genuinely concerned for The Doctor's well-being. Despite all his boasting, it almost seems as if The Master doesn't have the heart (or hearts) to kill The Doctor.
Later, there is a mesmerizing scene where The Master struts into the room which houses the Keller Machine and tries to impose his will over the machine. Part of the reason for this is to try and control the machine for his own use, but mostly, you can almost sense the resentment from The Master that he is unable to break the will of the machine. It's not the fact that the machine resisted him that bothers him. It's the fact that it dared to resist him that upsets him the most.
Once The Master fails to control the machine, he endures a similar experience that The Doctor and the other less fortunate victims of the machine went through. However, this time, The Master is only haunted by one image - a giant visage of The Doctor, laughing and mocking The Master from on high. It could have simply been a case of the fact that since we know so little of The Master, any images from his past would be unfamiliar to the viewer, and thus The Master's fear would seem meaningless (although I secretly wish that The Master had a deep rooted phobia of clowns or kittens, meaning we would have seen cheap photograph cutouts of adorable cats flying around the screen).
I prefer to think that there is clearly a past to this Doctor/Master relationship that we don't know about (and, thankfully, we never know about in great detail). But the fact that The Master fears the mockery of The Doctor, while The Doctor doesn't seem to fear The Master at all, is worth thinking about. Was there a time when The Doctor, for all intents and purposes, was a bully to The Master? Was The Doctor the more evil and immoral of the two? Was the evil of The Master a direct result of The Doctor's actions?*
*Only time, and an unfortunate flashback sequence in the new series episode The Sound of Drums, will tell.