Why did the Sixth Doctor regenerate?

By the end she was so sick of him, he repelled her so much that she gave up. He didn't want her carrot juice, didn't want her exercise videos, so she just decided to let him do what he wanted. She let him stuff his face with burgers and milkshakes. She let him lie for days, barely moving, just eating and watching television. By the end he reminded her of Elvis Presley in 1977.

Of course, his hearts gave way. They just couldn't take any more abuse.

The regenerated version was much improved.

- Carrot Juice

After my philosophical treatise Is The Master The War Chief? (short answer: duh, obviously...), Is Jack The Face of Boe? (short answer: no), Is There A Season 6b? (short answer: meh, probably, but The Two Doctors isn't part of it) and Am I Turning Into A Complete Jerk? (short answer: reply hazy, try again later), I turn my mighty mind onto this most vexing of questions...

WHY THE HELL DID OL' SIXIE DIE?!?

"I’ve come to think of him as invulnerable. Yet you saw him die — one of him, at least. How did it happen?"
Mel pursed her lips. "I didn’t actually see it. I was unconscious at the time. But I think..."
"Yes?"
"Well, he fell over and banged his head on the TARDIS console."
Benny laughed until her sides ached.

- Head Games

There are, of course, numerous takes on this. Some of them my own. It's generally assumed something along the lines of this happened: the Sixth Doctor and Mel are having an awesome adventure, the Doctor gets mortally wounded and dragged back to the TARDIS which then gets hijacked by the Rani. It's a nice idea, isn't it? Everyone goes for it, from the creme de le BBC Books range (Spiral Scratch by Gary Russell) to the lowest of fan fic hacks (Kronos Square by Ron Mallett)...

In the console room of the TARDIS, Mel stared at the nearly dead body of the Doctor lying on the floor. He had set the TARDIS moving. Peering at what controls she did know,she saw that the destination was Gallifrey. "Peri..." came the weak voice of the Doctor. "Going soon. Time to say..."

She ran next to him. "I'm not Peri. I'm Mel. What's wrong, Doctor?"

"This is not the end,but it has been prepared for..."

"What? You're not making any sense,answer me Doctor!" Mel wanted to shake the Doctor back to his senses, but she feared that the shock would kill him.

"Tears, Sarah Jane?" his face strained to form an emotion.

At that point, Mel sat back and began to think. He was a Time Lord. He had amazing powers of regeneration.

"Nooo... nooooo... Stop it,you're making me dizzy...... noooo..."

She made up her mind at that point to try to shock the Doctor into a regenerative cycle.

She moved to shake the Doctor, but the TARDIS began to violently shake. She fell back and knocked her head on the console. Mel fell to the floor sprawled out. She fell to the floor, knocked out. The shaking rolled the Doctor onto his stomach.

"This old body of mine is getting tired..."

And then he fell unconscious from the strain. The TARDIS materialised. The door opened, and two beings entered in. One pointed to the Doctor, and the other one reached down to the limp form. As he flipped the Doctor over, the regeneration began.

- Century's End

...but they all miss a whacking great, epic point.

MEL: I know about regeneration, of course.

Mel is not expecting the Doctor to regenerate. She doesn't even think he's been injured.

MEL: You claim I was alone when you found me.
IKONA: Oh, don't go on about this Doctor again.
MEL: I have to.
IKONA: There was no one else in the strange box. If he exists, he must have left you.
MEL: No way! The Doctor wouldn't do that.

It's rather hard to fit with scenes of Mel weeping over the dying Sixth Doctor, isn't it? And given her inhumanly good memory, you think she wouldn't FORGET something like this - as far as Mel is concerned, the Sixth Doctor is alive and well. She's not worried that, say a regenerated Doctor has wandered off while she was unconcious, is she?

MEL: You're nothing like him. If the Doctor's been harmed...

She gives absolutely no hint that when she last the saw the Doctor he was at death's door from chronon-blood-loss or being zapped by a Sleeper or doing a strange deal with the White Guardian to regenerate into someone badass...

"Hogwash! I don't believe in destiny! All this rubbish about me being The Ch'ar'K'yrt, the chosen one. I am not some reincarnated Rassillon."

"Doctor, you know that at the next regeneration The Change will come, the Champion will be born. I have reset the TARDIS coordinates, it will take you to the point of change, goodbye my friend and good luck." With that the White Guardian vanished.

"Goodbye." The Doctor whispered back. The two of them went back into the TARDIS.

"Where are we going?"

"Well the coordinates seem to be set for a planet called Lakertya..."

- Sacrifice

The novelization, by the authors of the teleplay (who, remember, are best mates with Colin Baker and dearly despair his lack of grand exit), make this clear...

(The Sixth Doctor is brooding over the console. Mel is skipping in the corner.)

MEL: 52, 53, 54...
SIXTH DOCTOR: Stop skipping, Mel!
MEL: Doctor, just because you don't object to being overweight is no reason why I should...
SIXTH DOCTOR: Don't argue! Stop!

(Mel does so and joins the Doctor at the console.)

MEL: What is it?
SIXTH DOCTOR: I don't know. (runs hand through hair) The slide control for setting time and space co-ordinates seems to be stuck...
MEL: (checks controls) This isn't operational either...
SIXTH DOCTOR: Take a look at the computer read-out screen.

(Mel does so.)

MEL: Blank! I'll run a check on the circuit...

(Mel starts pressing buttons to no avail. The Doctor suddenly looks up in horror and runs around the console to press a control, but before he can the room lurches and Mel is flung to the floor.)

MEL: What's happening, Doctor! What's happening?

(The room lurches again, flinging Mel first against the wall then the console. The Doctor tries to rise, but falls and cracks his head against the console plinth. He slumps, as does Mel. The exercise bike collapses, and then the room falls still. The doors open and the Rani enters.)

I'm not saying it's a version I'd be proud to call canon, but it's Word of God, people: the Sixth Doc and Mel were minding their own business when the TARDIS crashed.

Having this dynamic redhead as a companion was a prospect he viewed with unqualified pleasure. But that pleasure was to be tempered by a hazardous journey into uncharted territory. Hazards that were destined to have a profound impact on the Sixth Doctor.

For he was about to embark upon a series of adventures that would eventually culminate in a confrontation with the Rani. After which, this Doctor would never be the same again...

- The Ultimate Foe

That so many say otherwise as though all these authors (and, hell, I've done it myself) aren't paying attention to anything in Time and the Rani post the opening credits. (Though I have the excuse that I only recently saw the full story).

"Here they come!" The Doctor shouted, he rubbed his cat pendant for luck and then set his hands flashing across the controls of the TARDIS.

The time rotor started to move just as the proton missiles reached them. There was massive impact and everything in the TARDIS was tossed about as the TARDIS dematerialised. The Doctor and Mel lay unconscious on the floor...

- Kronos Squared

Now, some of the NAs tried to work with this...

Death laughed, and raised a soft finger between the Doctor's eyes. 'Don't lie to me! You sacrificed the colourful jester because you needed to be born! Time would have her champion, and he was just the compost for your blooming. You ran your TARDIS into the Rani's beam joyfully. Hah! Your sixth self hates you for that, he will become Valeyard for that –'

- Love and War

Which people treat as gospel. Ignoring the fact the above is
a) a dream
b) a dream by Ace of all people
c) contradicts the TV evidence as the Rani and the Doctor note that he had no idea about her operation on Lakertya and couldn't have dived into the "beam" anyway
d) written by an author that single handedly ensured all the NAs aren't canon anyway - BWAHAHAHHHHH...

So. To find out what really happened, we have to take in some curious facts, not least that the Sixth Doctor was alive and well before the TARDIS crash but clearly dead as a doornail afterwards.

• The TARDIS tool kit (easily identifiable as the one the Doctor's used since Earthshock) is open and positioned just next to the Doctor, who is last seen lying underneath the console, as if he was doing some traditional TARDIS maintenance when the disaster struck.
• The Rani's space harpoon is described by the dazed Seventh Doctor as "a navigational guidance system distorter" that "would force any passing spaceship into landing here". It sounds like its some kind of computerized siren that lures prey by reprogramming its flight computer... and NOT firing multicoloured laser beams from a planet's surface into deep space.
• The TARDIS is dragged down to the surface of Lakertya in a waterfall-like beam of light, very similar to how the Time Lords captured the ship in the previous story.
• In The Mark of the Rani, the titular Time Lady is unimpressed at the Master's ability to override the Doctor's TARDIS and draw him to a specific location. She herself can remote control her own TARDIS, a feat that impresses the Master and the Doctor. Now, if a hack like the Master can drag the TARDIS across the English Channel without so much as putting the wind up Peri's skirts, it seems a bit odd that the Rani can't do likewise.
• The Rani says, "I didn't go to the trouble of bringing you here just to discuss the ethics of my work," making it seem that capturing the Doctor and Mel was a lot harder than just firing the space harpoon into the sky.
• Mel notes that the TARDIS was "hijacked" by someone, rather than assuming it was, say, "shot out of the sky" or hit by an asteroid or simply went haywire. So the Doctor and Mel were aware someone was trying to take over the ship well BEFORE the rainbow fireworks struck.
• Mel states twice that she has no idea who the Rani is, so not only have they never met (even in the wonderful 53 years of missing adventures), the Doctor's never even mentioned her. Yet the Rani can not only disguise herself as Mel in moments, she can even do an incredibly accurate impersonation.
• The previous Rani story had the Doctor note her anal retentiveness meant she was incapable of doing ANYTHING "at random". She plans things meticulously. Are we supposed to think she has a wardrobe of Mel wigs and outfits to hand, along with amnesia drugs and the like just lying about the place and decided "what the hell" and used them all?
• Furthermore, since she needs the Doctor alive and well for her plans, why would she draw the TARDIS down in such a way it might (somehow) kill him in the process?
• The Rani shows absolutely no surprise that the Doctor has regenerated upon arrival, even though surely this means he is medically unfit for her services with his DNA in flux and his brain defences down. She never says anything like "I managed to work my plans around your regeneration", or tell him off for living so dangerously. In fact, she's surprised when he recovers so quickly and needs to drug him.
• If, say, the TARDIS landing was so violent it killed the Doctor... why wasn't Mel a sticky puddle of blood and bone in the corner?

So... this is my theory about what happened prior to the beginning of episode one.

The Rani has built the giant brain and wired in King Solomon, Hypatia, Za Panto, Charles Darwin, Louis Pasteur, Za Panato, Ari Centos, Niels Bohr, Steven Hawking and Jeremy Clarkson. Suddenly, part of the giant brain life support system explodes due to shitty wiring. She needs it fixed and fixed good.

The Rani's already decided to add Albert Einstein and the Doctor to the Giant Brain, and the Doctor can fix the equipment. Taking her TARDIS to kidnap the former, she spies on the latter for a while. Now, we all know that Time Lords have mental defences (The Brain of Morbius) and said defences are down during regeneration (Castrovalva). The Rani is also a brilliant neurochemist and TARDIS mechanic. Thus, she returns to Lakertya and constructs some kind of remote regeneration gizmo that will kill the Doctor and change his body, leaving him alive and vulnerable for her to dope, bluff and finally jam into the giant brain.

Using her navigational guidance system distorer, the Rani hijacks the TARDIS in the middle of a fitness session between the Sixth Doctor and Mel. Having five decades more experience, the Doctor gets out the tool kit and struggles to regain control of the TARDIS. With Mel's help, they succeed more or less.

Realizing they're getting away, the Rani uses her orbiting space satellites (the ones she uses to regularly keep an eye on the Strange Matter asteroid) to open fire on the TARDIS, simultaneously zapping the Doctor in such a way he'll regenerate on the way down. The turbulence knocks both the Doctor and Mel unconscious. The TARDIS is dumped in the quarry not far from where the Rani and Urak are standing with a TARDIS lockpick to break inside just in time for the Sixth Doctor to finally succumb to the artificial regeneration, which is why he recovers so quickly.

The novelization itself notes...

The prehensile claw It tugged roughly at the Time Lord's shoulder, rolling him onto his back so that he was face up. Face up? But these were not the rotund features of the Doctor. Could this be the endearing sixth Time Lord?

The Rani had no doubt. A single look was all the confirmation she needed. And she would not be mistaken.

She's expecting a new Doctor. She killed the Sixth Doctor to help her plans.

RANI: Am I expected to abandon my research because of the side effects on inferior species? Are you prepared to abandon walking in case you squash an insect under foot?

Of course, how did she engineer this to happen?

There was something wrong. Without moving an inch, the Doctor mentally checked himself over. There was definitely something wrong. His body had suffered a mental and physical battering from the forces that had attacked the TARDIS. The symbiotic link that he enjoyed with the sensitive consciousness buried deep within his ship had communicated much of the stress of the bombardment to him, and his alien physiognomy had interpreted the attack in perhaps the only way it could.

He felt a by now familiar rush of blood to his brain, the opening of normally dormant glands to release unique Gallifreyan chemicals into his system. It was happenning again. His Time Lord genes had responded automatically to the assault and he was on the verge of regenerating for the sixth time.

The chemicals flooding through the Doctor's system made it hard for him to think. All at once, something grasped his side and pulled him over onto his back before the final neural switches tripped and his regenerated started in earnest...

- The Seventh Doctor Handbook

So, the regeneration was psychosomatic, like the way Sil tried to kill the Doctor in Vengeance on Varos. The Doctor felt the TARDIS' pain, his mind thought he was dying of being zapped, his body agreed...

His alter ego smiled. "Dead? Oh no, Doctor. I’ve been waiting. Waiting a very long time for you. I’ve watched your pathetic meanderings through time, your meaningless battles. I saw you taken in by that scheming harpy the Rani, saw you die again. How did you ever let yourself be fooled by her?"

- Matrix

Thus, the Sixth Doctor was deprived a heroic exit, the last thing he managed to do was tinker with the console before being knocked unconscious and forced by the Rani to regenerate just to help with her scheme.

Ergo, the Sixth Doctor's comments in Zagreus are completely accurate!

SIXTH DOCTOR: Not bad, I suppose. A little overwrought, perhaps? But it's a better exit than I ever had! A bang on the head I ask you...