This is part of my series of essays concerning the In-Depth History of Doctor Who.
Coming Soon: Chronology of the Fifth Doctor's Adventures.
It was 1982. The 19th season of Doctor Who was about to begin and things were going to be quite different in the 80s. We now had a new scarf-less, cricket playing Time Lord, the first true Doctor of what some have called the "costume era" of the show, when our hero seemed far less willing to change his clothes from time to time.
NEW BEGINNINGS
When last we'd seen the Doctor, he had teamed-up with his arch-enemy the Master to save the universe from its own death throes. Though they succeeded, the Master had turned on his ally and used the chaos to hold the universe hostage. The Doctor foiled his schemes but in the process he wound up falling over a hundred feet to the ground below. Surrounded by his friends, he had reached out to "the Watcher", a projection of the man he was yet to become. And then, as if had happened before, he regenerated, his body rebuilding itself into a new form with a new face.
The new TV adventure "Castrovalva" was the Fifth Doctor's true debut and we saw a side of the Time Lord we hadn't been exposed to very much, that of a vulnerable man. Due to the severe all-around damage he had suffered from his deadly plummet to the ground, the Doctor was having a very difficult time regenerating. He wandered around the TARDIS for several minutes, unraveling the scarf he'd been wearing during his previous death.

When his young companion Adric finally found him, his memories were still scattered and he was not sure of himself. He simply smiled to the boy, as if meeting him for the first time, and introduced himself.
FIFTH DOCTOR: "Ah! Welcome aboard! I'm the Doctor ... Or at least, I will be if this regeneration works out."
The Fifth Doctor wandered around, trying to get his bearings. This was a great concern for new companion Teegan Jovanka, who had only stumbled into the TARDIS by accident during the previous adventure "Logopolis" and was still coming to terms with the idea that Adric, Nyssa and the Doctor were all aliens. The further revelation that the Doctor could change his appearance and aspects of his behavior yet still be, at his core, the same man was a bit much for her. Yet Nyssa explained to Tegan (and to anyone in the audience who shared the concern) that despite appearances, this was still the Doctor and he just needed time to settle into his new form.
As always, the Doctor was startled by his new appearance. Finding a full-length mirror near the TARDIS's wardrobe room, the Fifth incarnation looked at his new youthful face with concern and possibly disappointment, muttering, "That’s the trouble with regeneration. You never know what you're going to get!"
Wandering through the wardrobe, the Fifth Doctor found a recorder that had belonged to his second incarnation. Dismissing the instrument, he outfitted himself in what looked to be a stylized version of a casual cricketing outfit. Just as producer Jonathan Nathan-Turner had made Tom Baker wear question marks on his collar during the previous Doctor's final season, Peter Davison did the same.
Finally, he reached the Zero Room, a place in the TARDIS we had never heard of before but which he expalined was a safe zone where one was shielded from all outside influences, including radio transmissions, energy fields and background radiation. This reminded TV audiences that though he was superior to a human being in many ways, the Doctor was also vulnerable to subtle things we did not percieve or detect as interference. Along these same lines, the Third Doctor had mentioned a deathly allergy to aspirin and the Ninth Doctor years later would suffer great psychic pain from entering a room that had belonged to a terrified, inhuman child.
In the calming atmosphere of the Zero Room, the Fifth Doctor's true personality became more apparent. He spoke to Teegan and Nyssa openly and honestly as a man who was laid back and good-humored but also clearly in charge. He explained that he just needed to spend some time here and that the two of them, working with Adric, would be able to pilot the TARDIS for a little while without him. But things were not to go smoothly, since Adric had been kidnapped by the Master.
By the end of "Castrovalva", the Fifth Doctor had proven to his companions (and to many in the audience) that he was still the clever Time Lord he'd always been, foiling another plan of the Master's as he rescued Adric. The Master escaped capture but the Doctor was in too high spirits to let that bother him much. He was healthy again and looking forward to the future. As he and his companions journeyed back to the TARDIS to begin the new season in full force, the Fifth Doctor reached into the inner pocket of the jacket and produced a piece of celery that he then stuck on his lapel.
Jonathan Nathan-Turner had no real reason behind this decorative vegetable. He just felt that the Doctor should have something strange with his outfit and apparently dressing him in a cricketing outfit with question marks on his lapel just wasn't odd enough. Thus, the Fifth Doctor walked around with a celery on his coat that no one in the stories seemed to really question. This was another reason why some people criticized Turner's run as the "costume era."
Fun trivia note: Davison hates celery and couldn't stand it when in "Castrovalva" he had to eat the vegetable in a scene that took several takes.
A few years later, in his last adventure, the Fifth Doctor finally did come up with a reason for the celery when his latest companion Peri Brown asked about it.
PERI: "Doctor, why DO you wear a piece of celery?"
THE DOCTOR: "Safety precaution. I'm allergic to certain gases in the praxis range of the spectrum."
PERI: "So how does the celery help?"
THE DOCTOR: "When the gas is present, the celery turns purple."
PERI: "And then what happens?"
THE DOCTOR: "… I eat the celery. If nothing else, I'm sure it's good for my teeth."
Many years after that, in the audio play "The Judgment of Isskaar" produced in 2005, the Fifth Doctor was palling around with a girl named Amy who remarked on his question mark collar.
AMY (impressed): "So you know where we are this time? ... You should write a book or something."
DOCTOR: "And give away all my secrets? I like to retain a certain hint of mystery."
AMY: "Hence the symbols on your collar. 'I'm the Doctor! I'm the man of mystery!' "
DOCTOR: "You know, you got a lot more sarcastic since we first met."
AMY: "I got a good teacher."
So now, let's talk about this new actor who was in the star role.
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