A battle against the Cybermen in the South Pole took its toll on the increasingly frail Time Lord, who lapsed in and out of consciousness while trying to fend off the Mondasian threat. He was taken captive by the Cybermen and held inside their spaceship, but had enough presence of mind to allow the invaders to drain too much power from the Earth and destroy their home planet.
In front of companions Ben and Polly, the grouchy white-haired time traveller collapsed inside the Tardis and transformed from the ailing William Hartnell into the endearing cosmic hobo Patrick Troughton.
Recent adventures have displayed The Doctor's fondness for his Time Lord race, but they didn't do his second incarnation many favours. In fact, they found him guilty of interfering in the affairs on other planets and exiled him on Earth - along with forcing a regeneration upon him. The last moments of Patrick Troughton's time traveller were as a disembodied head floating away in a dark void. The next time we saw The Doctor - in glorious colour - occurred when Jon Pertwee stumbled out of the Tardis and collapsed.
Big spiders send shivers down the spines of many folks, but rarely do they make anyone regenerate. The Doctor's indirect destruction of angry arachnid 'The Great One' on Metebelis 3 caused a huge wave of radiation to fill its cave after it was tricked into using a crystal for its evil purposes. This radiation weakened The Doctor and affected his cells, although he made it back to UNIT headquarters. He regenerated in front of the Brigadier, Sarah Jane and Harry Sullivan and quickly bounded around with a radically eccentric personality and jelly baby fetish.
Video killed the radio star, but it was a radio telescope that curtailed the Fourth Doctor's reign. A bid to unplug a power cable and prevent The Master from sending out a universal message from the telescope caused The Doctor's mortal foe to tilt the big satellite dish and leave him dangling high above the ground. Intensely staring straight ahead, The Doctor kept hanging on while experiencing visions of old enemies like the Daleks urging him to die. Then, suddenly, he lost his grip and fell to the ground.
"It's the end," he told his companions, "but the moment has been prepared for." Out of nowhere, the mysterious figure of The Watcher moved over to The Doctor, merged bodies with him and transformed into the younger Peter Davison. Bafflingly, it transpired that The Watcher was the future Doctor all along.
Everything seemed to be going well for this youthful and sprightly incarnation until he paid a major price by saving the life of his dying companion Peri. They were both suffering from spectrox toxaemia, and the only cure was bat's milk located in an oxygen-free cave deep down in the planet Androzani. It's certainly not something you can pick up at your local Tesco. Sadly, there was only enough milk for Peri and she could only look on as her friend collapsed on the Tardis floor.
In a memorably tense scene, The Doctor experienced hallucinations featuring a range of former friends and foes, who either implored him to survive or die. The Master was particularly vocal in his desire for The Doctor to push up the daisies, but suddenly our hero's head jolted up from the Tardis floor - complete with blond afro and an inconspicuous blink. "Change my dear, and it seems not a moment too soon," the arrogant new figure stated.
After Colin Baker was harshly sacked from the role and refused to return for a regeneration scene, Sylvester McCoy had to wear a curly blond wig and be filmed from behind wearing the multicoloured Sixth Doctor outfit. Not exactly the most dignified start to a new era on the show.
An attack on the Tardis by a Time Lady called The Rani forced the vessel to crash land and The Doctor to regenerate from the resulting injuries. Somehow, companion Bonnie Langford emerged unscathed.
It wasn't any monsters from outer space that did for The Doc - it was his beloved human race! He emerged from the Tardis in San Francisco and was immediately pelted with a volley of bullets from some angry Triads. Welcome back to Earth. To make matters worse, Dr Grace Holloway ignored the Time Lord's protests and tried to operate on him despite his distinctly non-human internal organs. Two hearts beating together certainly weren't in Grace's medical textbooks.
Her efforts failed and the seemingly deceased man was locked away in a morgue, only to regenerate into a floppy haired and housewife-pleasing younger persona with a Jesus Christ complex and dubious fashion taste.
The exact circumstances regarding his regeneration are unknown, although it is strongly believed that he perished during the epic Time War. Evidence suggests that the Eighth Doctor regenerated shortly before his successor encountered the Autons and a certain chavette companion called Rose, as he notably comments on his new features when looking in the mirror at Jackie Tyler's flat.
A short-lived and undoubtedly 'fantastic' stint was brought to an end after The Doctor sacrificed himself, just like his fifth incarnation, to save his companion. Cellular degeneration and regeneration into Barcelona-bound David Tennant ensued once he sucked out and absorbed the Time Vortex from Rose Tyler's mouth. Has anyone else ever found a better pretext for a sloppy snog?
The Tenth Doctor
Who knows? Watch this space...