One of the most incredible processes that took place in the TARDIS was finally coming to an end – for the moment at least. When it did, Finite Occupancy would be reached.

The TARDIS raced through the Space/Time vortex. The Doctor wanted to show Amy the Delirium Archive, the Universe's biggest museum. As they waited to arrive yet another question came to her: 'So, when you regenerate, can you decide what you look like?' She was still in the early days of travelling with the Doctor. Of all the amazing things she'd discovered about him the most was his ability to change his body and personality. All the big questions about this – was it dangerous? How could she help if it happened? How long did he take to recover? etc. – were dealt with and, over the next few days, less serious ones kept springing to mind.

'What I look like?' the Doctor replied. 'Ooh, hold hard…' His face suddenly zoomed at a rack of meters on the TARDIS console. 'Oh no, that's quite alright.' He turned to Amy. 'No. Some of the Time Lords' biology allowed for a choice of looks, but not mine. It'd be awesome, though, wouldn't it? But no and, actually, both options are on the cards.'

'Boy or girl?' Amy cocked her head. No way. 'Shut –up!'

'Yep. Could be anything from a grumpy, sweary guy to a cosy, jam making old dear. Well, I say cosy, she'd probably have a fair bit of swagger – you know me. But, whatever, you need to be as prepared as a boy scout.'

'Or a girl guide. So, like, back in the day, what d'you look like?'

'One was an old fellah with long white hair,' the Doctor said. 'Forgetful, snappy, always up for a scrap. Back on Gallifrey he was given the equivalent of an ASBO. Another was young, blonde. He was uptight, diffident, obsessed with cricket.'

Amy made a face. She couldn't believe this. Cricket?' Boring'est – sport – ever! That was your thing?'

'That's the point, Pond. Keep up! It wasn't me. And, of course, conversely, it was. That was one of my less easy regenerations – a bit more than some manic moments and the brain dendrites all wobbly. Another tad difficult one was when I became this curly-wurly haired, hippy looking chap. His arrival coincided with a gang of techno-Nazis trying to blow up the world. I dealt with them and just hours later I had a blackout – weeklong, apparently. I went AWOL. My friends found me in a daze in Trafalgar Square with a goldfish in a bag. There's a few patches of memory – an interview at a Wimpy bar. A jaunt in the TARDIS, to 1581. Awesome afternoon, about as awesome as afternoons get. You can't beat chucking round a Frisbee with Sir Francis Drake… Anyway, my regeneration into me wasn't crazy – well, you saw that yourself – although, just before I came back for you, I did have a strange moment where I wanted to dress like a pirate. Did that for a few days.'

'You might have gotten over your peg leg and a parrot phase but are you sure you're not still regenerating?'

'What d'you mean?'

Amy smiled, a crinkle of sweet and sour. 'Got a mirror you wanna check out?'

'Ha ha. Tweed is cool. Bow ties are cool.' The Doctor proudly tweaked his. 'Like I've said before, this ain't going anywhere.'

A loud honking burst across the soft surge of the TARDIS engines. Oh, not good, Amy thought. Things are about to get crazy...'What is that?'

'Stay calm, nothing to worry about. It's the early warning signal that we're about to run low'ish on fuel.' The Doctor moved to the console's engineering panel, fussed a row of switches and the honking died.

'OK, so, where does a TARDIS fill up?' Amy said.

'Cardiff,' the Doctor said, re-joining her.

'Yeah, yeah, very funny.'

'Sorry?'

Amy laughed. 'Cardiff?'

'Yeah.'

'Wow, you've made even me speechless… Cardiff it is.'

The Doctor's head twitched. 'What's up with that?'

'Well, nothing, as it goes. You just don't expect a time machine to make a pit stop in Wales.'

'Object lesson in always expecting the unexpected, Pond – make sure you take that on board. Plenty more happens in Cardiff than anyone supposes. Well, actually, I find more happens everywhere than anyone supposes.' The Doctor laughed. 'Well, except on Orius Magna where dullness is in the DNA. So: Pit stop time. There's ages before the levels hit critical but you know how it goes around here, tonnes of stuff gets in the way…' He moved to the navigation panel and re-set the co-ordinates for Earth.

'So why Cardiff?'

'Above the city's airspace is a dimensional rift full of Artron energy, among other stuff. The TARDIS filters out what she doesn't need and then drinks in all the lovely Artron-y goodness. Takes about forty-eight hours.'

'Oh, here we go...' Amy said. A section of console to the Doctor's left had slid away, accompanied by the ring of an alarm clock. The warning must be less reliable than he thinks. 'Are you sure – '

'Oh, it's nothing to do with the fuel situation,' the Doctor said casually, his hands flapping. 'It's quite different. Yes, haven't seen this in a while…'He pointed and Amy joined him. What looked like a mini DVD player rose on a tripod from the space the console had revealed. A plaque in the lid read FINITE OCCUPANCY TERMINAL. 'Thank you, dear!' the Doctor called up to the ceiling. 'Message received! Roger – and – out!' He and touched a bell icon at the lid's top right. The alarm cut off and he opened the laptop. A message blinked on the screen, glinting letters of greenish copper: 'TIME TRAVEL CAPSULE HAS REACHED FINITE OCCUPANCY,' Amy read. 'IN ORDER TO RECOVER SPACE FOR FUTURE USE, SELECT BUILT ENVIRONMENT (ADDITIONS) FOLDER. SELECT MENU AND FOLLOW RETAIN/BOOKMARK/DELETE OPTIONS.' She frowned. Finite occupancy?

'This is one of the best things the TARDIS does!' the Doctor said, clapping his hands together and hurling a finger at the screen. 'Isn't she a honey? A percentage of the space in the TARDIS, Amy, is developmental; meaning, therefore, that she grows new rooms – and their contents, of course! – whatever she thinks her operator might like.'

'She can grow… stuff? And think?'

'Yes.'

'Wow, OK... How does that work?'

'Well, this wonderful old girl is a living thing. Well, up to a point – big organic component and a degree of sentience. And, all over her, are... pockets of potentiality, I s'pose you could call them: Spots of creative energy that adapt to the size of whatever the TARDIS puts there and tweak the surrounding architecture so nothing gets all squashed up. The process takes hundreds of years. And, when all the spots are filled, Finite Occupancy is reached. Then boom! The TARDIS is full.'

'But I thought it – she, you know, never… ended.'

'Never ended?' the Doctor said. 'No, not even the Time Lords' mastery of dimensional mechanics could achieve that. So, anyway, at Finite Occupancy I get a message so I can decide what to keep and what to chuck.'

'So, you lose rooms?'

'That's right. It means the architecture can be managed according to the operator's preferences. I'm not going to want everything she cooks up, you see… You have to watch the 'ole housekeeping. Drax – a friend of mine – once accidentally deleted all his loos. Made things, er, quite uncomfortable.'

Amy grimaced. 'Eugh… I'll bet.'

'Exactly. You might almost say he went from being able to go everywhere in Time and Space to having nowhere to go in his own TARDIS!'

'Oh I love ya Doctor, don't get me wrong; but I have to say, officially: 'Ooh, shocking.' And before you say it, I know, you're 'here all week.' '

'I know, I know, but I just couldn't help myself!'

'Just like your mate,' Amy said. 'But if you did wanna keep everything from a new batch you'd, what… Delete other bits?'

'Ooh, ace question, Pond – liking it,' the Doctor said. 'And yes, I could fiddle with other parts – dump other rooms I haven't used in ages.'

'Cool. It's like sorting your wardrobe. If you haven't worn something in six months, chuck it out.'

'Absolutely! Chuck – it – out! And, if there's something I'm in two minds about, I can always bookmark it to be re-made in the next lot of rooms.'

The engine surge fired up as the TARDIS began to materialize. 'Bora da, Cardiff,' the Doctor said. 'So: Let's re-fuel.' He moved to the engineering panel and flicked back a dial below two levers that always made Amy think of the gear stick of her Ford Focus. The sound of the engines faded and the Doctor pulled down the levers. 'Engine Filters open. Now, let's check out what she's come up with...' Going back to the Finite Occupancy terminal, he started to type. 'OK… Ah – Menu. Open… Blimey! Well, some of these are no-brainers. Aerodrome? Fifty extra miles of cloister? Don't need those...'

Forty–nine hours later…

Keep

Art studio

Back parlour

Crystallography lab

Operating theatre

Woodwork shop

Zero Room

Bookmark

Karaoke bar

Model village (super massive)

Pitches (cricket, croquet)

Scriptorium

Chuck

Aerodrome

Bakery

Bingo bar

Cloister (additional)

Drum circle

Gymnasium complex

Karaoke bar – B/M

Martial arts dojo

Model village (super massive) – B/M

Parlours (front/middle)

Pitches (cricket, croquet) – B/M

Scriptorium – B/M

'…ski slopes (dry), TV lounge and wine cellar. Yeah, happy with all those.' The Doctor closed his notebook and slipped it into his jacket. 'Sorry about all the re-checking, Amy, I want to make sure I've got this right.'

'No worries.'

'I have to say, this particular regeneration feels little bit more artistic than usual, so I'm glad I bookmarked the art studio. Anyway – keep.' The Doctor pressed a key, finally locking in the selections he'd made. 'It's a shame for my successor because he or she might well like some of these. But they're not here and I am…' He pressed a second key. 'Bookmark. Now, Amy, hold on.'

They both clutched at the console edge and the Doctor tapped a third key. 'Chuck.'

The room jolted and he and Amy steadied themselves. Above, from all around, she heard deep clicks and knocks and then a long, shambling crash. Like a rock fall, she thought. After thirty seconds or so it faded.

The Doctor and Amy let go of the console and he closed the terminal. He pushed down on the plaque and the computer fell away. 'And there we are,' the Doctor said. 'Ooh, forgot...' He took his pencil from behind his ear and pocketed it. 'You know what? Amy, I've had a change of mind! The Delirium Archive can wait. I wanna see something I've never seen before. So: The Fork Tongue nebula!'

'What's a nebula?'

'Giant gas cloud in space. This one's just to the left of Tau Ceti and filled with all different kinds of gases that mix together to create colours you just don't get on most worlds – should be really something to see.'

Amy nodded. 'Sounds awesome. Way cooler than a museum.'

'And you know what else? I really fancy a cuppa so I think we'll have a spot of tea as we check it out.' The Doctor moved to the navigation panel. 'So!' he said, throwing the GO! lever, 'I'll put some parsecs between here and there and you pop off and make up a thermos of tea.'

'OK.'

'And don't forget the Jammie Dodgers!'

Across the TARDIS, moments after the Doctor had shut down the Finite Occupancy terminal, rooms began to blossom.