"Mel seems to be doing well for herself," the Doctor smiled to himself as he walked back into the TARDIS. A shudder overtook him and he groaned, leaning against the doors he had just closed. "Who's next?" he grunted, and forced himself to stagger up to the controls. "Perivale, 1996 should do it." The Doctor flicked a few switches, pressed a button or two, and swung around the console to pull the lever that would bring the TARDIS into the Time Vortex.

Less than a minute later the blue police box materialized on a street corner in his desired destination. A young woman in her mid-twenties with long brown hair and a square jaw was rushing down the road but stopped in her tracks when she heard the noise. Could it be? A grin spread across her face as the TARDIS appeared. "Ace," she breathed, and cautiously stepped up to the doors.

Dorothy Gale McShane, or Ace as she still preferred to be called, pulled out an odd-looking key from her coat pocket. As she held it up to the lock, though, she realized with a frown that it would be much too big to fit into the slot. Suddenly, she watched in amazement as the key began to change in her hand. It glowed with faint gold energy and transformed into a simple Yale key. She pushed the key into the hole and carefully turned it, before entering the glorious time machine for the first time in three years.

"It's—it's changed," Ace gasped as she looked around. "It's all… coral-ly. Eh, I liked the old one better." She walked towards one of the big coral pillars and fingered the long, tan suede jacket that hung over the opening. "I wonder whose this is. The Doctor could never fit into it." She continued to look around, as she had not yet noticed the tall thin brown-haired man standing on the other side of the console.

"Ace?" a voice rang out. The woman in question spun around, and found herself looking at a man in trainers and a suit.

"Who are you? How do you know who I am?" Ace's eyes narrowed.

"You really don't recognize me?" he grinned, coming towards the young woman. Then his smile deepened into a frown. "Miss McShane! You're not still carrying Nitro-9 around, are you? And at this age!"

Ace faltered. "Professor?"

"It's 'Doctor,'" he said, but smiled again all the same.

"Oh, Doctor!" Ace cried, burrowing him in a quick, but all the same tight, hug. "You've gone and… oh what's the word you used… regenerated!"

"Three times since we've met, actually," he grinned.

"Wow, really?"

"Yep," he declared, popping the "p". "But seriously, why are you still carrying around Nitro-9?" the Doctor continued, gesturing to the large backpack she carried.

"Oh, I'm a weapons consultant for UNIT now," the twenty-five year old beamed. "They love my ideas." Noticing the Doctor's frown, however, Ace rolled her eyes. "You didn't really expect me to stay out of trouble, did you, Professor? And it's not like I let them use it for anything extremely dangerous."

"I just don't want you to get hurt," the Doctor said. Ace raised an eyebrow.

"Don't be gettin' all sentimental on me, Doctor," Ace warned. Then she smiled again. "I've also been thinking… UNIT's great but I want to be in charge of something. It's a little boring just being a weapons consultant. What do you think?"

"Oh Ace," the Doctor grinned. "You're going to be great. Brilliant. Fantastic, even."

Ace chuckled at his praise, but then became puzzled. "Is something wrong, Professor?"

The Doctor took a deep breath. "I'm regenerating again. But don't worry about me, I'll be fine. Go on, Dorothy McShane. Show the world what you're made of."

Ace would have corrected the Doctor on his use of her birth name, but given the current circumstances, she refrained. "Are you sure you'll be alright, Doctor?"

"Yes," the Doctor lied, leading Ace to the door.

"You're lying, aren't you." It wasn't a question.

"Now, how could I be the Doctor if I didn't do that?" he smirked. Ace now stood outside the TARDIS doors.

"Goodbye, Professor," she smiled.

"Goodbye Ace," he replied, and closed the doors. Ace was a ways away down the sidewalk when the doors swung open again, though now the Doctor was at the controls. "Oi, Ace!" She spun around to see him. "Don't go getting into trouble!"

"Try and stop me!" she laughed as the doors closed again and the TARDIS faded away.

I believe that after the Doctor Who special "Dimensions in Time," the Doctor had to explain to Ace what regeneration was, what with all those other Doctors running about. Secondly, in this particular story, the fanon is that the Doctor sent Ace back home soon after DiT.