This one requires a bit of an explanation, by way of a story:
The Doctor was far too engrossed in his experiment to notice the soft knock at the laboratory door. It was only when he heard a throat being cleared in no uncertain terms that he ceased his measurements, and took notice.
The voice was sweet, but unfamiliar. A woman stood patiently in the doorway, her hands folded. It seemed that the Brigadier had wasted no time in finding a replacement for Liz, in spite of the Doctor’s protests that he did not need to be… supervised. The woman looked as though she had been freshly plucked from 1918, in driving gloves and two strands of pearls, a heathered brown jacket over her pale dress, and a feather of purple and gold fanning out dramatically from her felt hat. The Doctor felt as though there was something uncanny about her freckled smile, but if he did know her, he could not place why.
“I’m sorry, have we met?” ventured the Doctor.
“Of course we have,” she replied. “It’s me, Bessie.”
“Oh, what a coincidence,” the Doctor smiled, extending his hand to her, “Bessie’s the name of my motorcar. Pleased to meet you.”
“Bless your beautiful little heart, no,” she giggled, punching him playfully on the arm. “You see, old thing, I am your motorcar.”
“You’re my motorcar,” he repeated.
“Beep beep,” she blushed.
The Doctor could see four distinct possibilities as to the nature of his current situation: either he had gone mad, or this charming young woman was, in actual fact, stark raving bonkers, the Brigadier had enlisted an intern in a misguided attempt to have a bit of fun at the Doctor’s expense, or he was speaking to his beloved roadster, whose consciousness had manifested itself in humanoid form.
“You don’t believe me, do you?” she asked. “Well. Two weeks ago, you swerved us right into a rubbish tip, do you remember?”
“Quite clearly, and I also remember that I’ve never told anyone about that,” squinted the Doctor.
“I know,” she grinned, standing far closer to him than what most Edwardian ladies he had known would consider acceptable personal space. “I was there. I was shaking crumbly brown rubbish out of my undercarriage for days!”
And so it was that Possibility Number Four – that she was, in fact, his Bessie – seemed the most ridiculous, and therefore the most likely to be true.
“Well, I’ll be,” he said quietly, studying her. “It is you, isn’t it?”
She held out her arms, pulling him into a hug so warm and so powerful that he was nearly winded by its force.
“And I have arms now! Isn’t that something?” she exclaimed.
The Doctor struggled to catch his breath when at last they broke apart. The old girl was stronger than she looked, he thought. She really was Bessie, after all.
“It was jolly strange, old chap,” she continued, “one minute I was sitting quietly in my garage, just thinking about how lovely it would be to go on a drive through some of those bits of country road around here where no one checks whether or not you’re going too fast, and then poof! I’ve got… legs, and a face, and no wheels.”
“Yes, but how did you do it?” puzzled the Doctor.
“How should I know? You’re the scientist, my dear,” she shrugged with a laugh. “I’m just your chauffeur.”