The big plan had been to go back in time and watch the Sistine Chapel being painted. However, hopes of rubbing shoulders with Michelangelo had been quickly dashed by matters such as the chapel labourer who had the worst B.O. imaginable (Bill retched) and a deranged pedlar who chased Bill down the streets for ten minutes after he spotted her watch. (The year 1509 is a pretty stupid time to be checking a watch.). The pedlar and the B.O. kind of killed the moment, so they left quickly and now they were back in the TARDIS, with Bill reading 1970s copies of The Beano and the Doctor fiddling about with stuff, muttering to himself now and then, and then pulling funny faces that were gradually getting funnier.

"I keep thinking I can smell fish," he said.

Bill looked up from her comic. "Oh. Have you got a secret pet goldfish?"

The Doctor laughed. "If only! No pet's charisma comes close to the charisma of the goldfish!"

Bill had started thinking. "So, if you could have any pet, what pet would you have?"

Without delay, the Doctor said, "Not a cat!"

Bill couldn't believe it. "What?! Not a cat? Cats can be your best friend!"

The Doctor laughed. "Ah! They've got you! They fooled you! Don't be fooled! They're only your best friend as long as you've got a can opener."

Bill said, "But what if you've got a can opener but no food?"

"That's simple," said the Doctor, "they'll go find a new best friend who has got food. Preferably lots of food. Or endless food. They don't mind, as long as it's food."

Bill said, "But my next door neighbour Betty's cat loved me and I didn't ever give it food."

The Doctor frowned and said, "The owner was old, wasn't she?"

Bill's mouth dropped open. "How do you know that, Sherlock?!"

The Doctor said, "The cat was checking you out as a potential future food source for when the old lady went to the big cattery in the sky. If she was young the cat would only have been interested in you if you had…wait for it…food. And don't be fooled into thinking cats love you. They only love food. And can openers."

Bill said, "You're harsh on our furry friends! You make them sound so conniving and calculated."

The Doctor laughed. "Oh yes, I totally expect to have to save you humans from the cat takeover at some time in the future. Watch them...they're waiting."

Bill said, "So, if you could have any pet, what pet would you have?"

The Doctor said, "Not a cat!"

Bill said, "What?! Not a cat? Cats can be your best friend!"

The Doctor said. "Ah! They've got you! Don't be fooled! They're only your best friend as long as you've got a can opener."

Bill said, "But what if you've got a can opener but no food?"

The Doctor didn't answer.

The Doctor and Bill just looked at each other.

For quite a while.

Silently.

Looking confused.

Thinking.

Bill broke the silence at last.

"What was that? Did we just…repeat ourselves?"

"We did," said the Doctor, with a deep frown starting to creep across his face.

Bill said, "What on earth happened there? I mean WHAT ON EARTH happened there? We just repeated a whole chunk of conversation and it felt like I wasn't in control of what I was saying."

The Doctor took a deep breath. "You know how they say that when you face death that your life flashes before you?"

Bill said, "Yes, I've heard that."

The Doctor said, "Something like that is happening. Though in space it's different rules. In space it's a warning. Minor things – like speech and small actions – repeating themselves are a big big warning. And unfortunately it's the kind of warning that can only mean one thing."

Bill looked nervous. "What's that?"

The Doctor said, "We will both, most surely, soon die."

Bill's face said what she was thinking: FEAR.

The Doctor said, "I have some good news."

Bill said, "What's that?"

The Doctor said, "I didn't say any of that."

Bill looked confused. "What do you mean?"

The Doctor said, "All that stuff about dying. It was total nonsense. None of it was said by me."

Bill said, "So what were you trying to say?"

The Doctor said, "I shoplifted a pair of bloomers from Marks and Spencer's once."

Bill said, "Huh? Sorry mate. But what? If there's a person in the Guinness Book of Records who's listed as being the most confused person on the planet then I'm a bit more confused than them. I'm living in confused-ville right now."

The Doctor sighed. "I didn't say that either!" The Doctor paused then added, "But I know who did!"

The Doctor paced for a few moments. "What to do? Hmm. Formulate a plan. Execute a plan. Formulate a plan. Execute a plan. But what?"

The Doctor was pacing and thinking deeply for a minute when finally he clapped his hands, jumped up and shouted, "Aha!" He turned to Bill and said, "What did Archimedes say in the bath?"

Bill said, "Was it…'blimey, I'm getting out of here, it's bloody cold'?"

The Doctor laughed and put his hands on Bill's shoulders. "Don't worry. I know you didn't say that. 'Eureka' is the answer. As you know. And I know you tried to say that but other stuff came out."

The Doctor did a little jig. "I know what to do! Oh yes! I know what to do!."

Bill woke up and rubbed her head and let out a little groan. She looked and saw she was on the floor. Ouch, her head hurt! She looked round the TARDIS. She frowned and looked across the TARDIS and spotted the Doctor, who was sat down and smiling smugly.

She asked, "What happened? One minute we were talking about cats and then weird stuff happened. Before I knew it we were talking about bloomers and then you had a plan and the next thing I know is I wake up and I have a painful bump on my head."

The Doctor smiled. "You just said that. No repetition. No nonsense. You said exactly what you wanted to say. That's good to see. However…I really must apologise for that bump."

Bill looked confused. "This is where I ask you what on earth happened, right?"

The Doctor put his hand on Bill's shoulder and said, "We're back to normal now. We can say what we want without any silly interruptions."

Bill said, "Interrupted by who? Or what?"

The Doctor said, "Amuatic gas."

Bill looked confused. "Amuatic gas? Fill me in."

The Doctor took a deep breath. "Amuatic gas is a living organism. It's basically gas that's alive. You heard that right – gas that's alive. And it can manipulate other gases – such as the air - even up to the point where it can change sound waves travelling in the air and distort the sound to make it look like the person is saying something they're not saying. So the poor person ends up saying something they never said and it can cause all manner of problems. Amuatic gases never tire of messing with your head. Your awkwardness and suffering pleases them greatly"

Bill said, "It doesn't sound like my kind of gas! Where did it come from?"

The Doctor filled Bill in. "A very clever scientist on the planet Drei – a scientific community planet – experimented with both artificial intelligence and creating gaseous life forms. Then, one day, he very un-cleverly merged the two projects and produced Amuatic gas, totally by accident."

Bill said, "Not smart."

The Doctor nodded. "Agreed. Amuatic gas is just as alive as you or me, though it's not very bright. Not very intelligent at all. It's kind of like the village idiot's dimmer brother. But a sociopathic dimmer brother that takes glee in the confusion of others. Fortunately, the scientist only made two."

Bill said, "Right. So basically, we've just been gaslighted by…er…a gas?"

The Doctor said, "Correct! But your friendly Doctor knew exactly what to do. Inspiration struck, but when it struck I didn't need to run down the street naked."

Bill said, "What did you do?"

The Doctor beamed. "I flooded the TARDIS with carbon dioxide. That, of course, meant it was "goodnight Bill" for a while - poor you- and thus you dropped to the floor like that well known sack of potatoes. Then I opened the door. As the CO2 exited the TARDIS the greedy – but stupid - gas thought the movement was an exhalation from a bigger organism and it went outside to go to the source. At that point I performed a very technical, highly complex scientific move."

Bill said, "What's that?"

The Doctor said, "It's called 'slamming the door'."

Bill was starting to look relieved. "So it's gone?"

The Doctor said, "Yes, it's gone. It's out there somewhere right now, but it won't be bothering us again any time soon."

Bill said, "But why me?"

The Doctor said, "The Amuatic gas was attracted to the CO2 in your breath."

Bill said, "So why not you?"

The Doctor said, "The CO2 that humans produce in the breath is a perfect environment for them. And it's just the right temperature. The gas chose you to be the host, Bill. Plus, that position by the mouth puts them in a perfect position to play their silly games and distort the air to make it seem like the person is saying whatever the Amuatic gas wants them to say. It only needs one host to distort EVERYTHING said by anyone in that room. You were that host. And it must also be said…brace yourself here…that they are only attracted to the gullible."

Bill couldn't believe it. "What?! Gullible? But why did it think I'm gullible?"

The Doctor laughed. "Because you are human. And humans are very gullible."

Bill said, "And you think humans are gullible? Thanks a lot mate! Et Tu, Brute! I think I preferred it when you were talking about bloomers."

The Doctor said, "Yes. Gullible. Please don't ask for evidence that humans are gullible . I haven't got all night."

Bill looked out of the Tardis. "It's always night out here."

The Doctor playfully punched Bill on the arm. "You got it, Billy!"

The Doctor was ready to round up. "So there you go, Bill, you've met Amuatic gas. Let's hope it's the last time. Those Amuatic gases, eh? They play games with your mind, they cause trouble and strife, they take glee in your confusion and – worst of all - they have no conscience to keep them in line. But I have to say this: There's one thing I really hate more than anything about the Amuatic gases. By a long shot, much much more than anything else."

Bill said, "What's that?"

"They stink of fish."