Martha waited for the Doctor in a back alley by the ATMOS factory. She turned upon hearing a whooshing to see the TARDIS appear behind her and smiled. The door squeaked and the Doctor stepped out after looking both ways.

"Martha Jones," he said quietly.

"Doctor," she replied. He took in her black leather jacket. They slowly stepped towards one another, then jumped into a friendly squeeze.

"You haven't changed a bit," he said with a grin.

"Neither have you," she responeded.

"How's the family?"

"You know, not so bad. Recovering."

"What about you?" Martha spotted a redheaded woman approaching.

"Right. I should've known. Didn't take you long to replace me and Angel then."

"Now, don't start fighting. Marth, Donna. Donna, Martha. Please don't fight. I can't bear fighting."

Donna chuckled under her breath as she approached. "You wish," she whispered before smiling to Martha. "I've heard all about you. He talks about you all the time."

"I dread to think."

"No, no, no. No. He says nice things, good things. Nice things. Really good things."

"Oh, my God! He's told you everything," Martha said as she brushed her hair away from her face. Donna spotted her engagement ring.

"Didn't take you long to get over it, though. Who's the lucky man?"

"What man? The lucky what?" The Doctor asked, totally oblivious.

"She's engaged, you prawn," Donna said as Martha waved her hand in view.

"Really? Who to?"

"Tom. That Tom Milligan. He's in paediatrics. Working out in Africa right now, and yes. I know, I've got a doctor who disappears off to distant places. Tell me about it."

"Is he skinny?" Donna asked.

"No. He's sort of strong," Martha said with a blush.

"He is too skinny for words. You give him a hug, you get a paper cut," Donna joked about the Doctor. Martha laughed and looked up at him.

"Oh. I thought you were fighting."

"No. Speaking of which…" Martha's walkie-talkie's sound comes back on.

"Dr. Jones, report to base, please. Over."

"This is Dr. Jones. Operation Blue Sky is go, go, go." Martha turns her heal and struts on. "I repeat, this is a go." The three of them stand and observe UNIT soldiers and vehicles approaching.

"Unified Intelligence Task Force. Raise that barrier now. Leave you safeties on, lads. It's non-hostiles," they heard a soldier yell. Then they heard, "All workers, lay down your tools and surrender," over a loudspeaker.

"Greyhound 16 to Trap One, B Section. Go, go, go! Search the ground floor. Grid pattern Delta," Martha directed through the hallabolou.

"What are you searching for?" The Doctor asked.

"Illegal aliens."

"This is a UNIT operation. All workers lay down your tools and surrender immediately."

"B Section mobilized. E Section, F Section, on my command," Martha shouted through the walkie-talkie and ran to position.

"Is that what you did to her? Turned her into a soldier?" Donna accused the Doctor. He had nothing to retaliate with and looked away.

"A Section, in position," yelled a soldier. Another yelled, "We are on standby, sir."

"You're qualified now? You're a proper doctor?" The Doctor asked Martha when he spotted her ID upon her return.

"UNIT rushed it through, given my experience in the field. Here we go. We're establishing a field base on site. They're dying to meet you."

"Wish I could say the same," the Doctor muttered.

"Operation Blue Sky complete, sir. Thanks for letting me take the lead. And this is the Doctor. Doctor, Colonel Mace." The Colonel swiftly gave the Doctor a salute.

"Oh, don't salute," the Doctor implored with a frown.

"But it's an honour, sir. I've read all the files on you. Technically speaking, you're still on staff." The Colonel told the Doctor, who decided to roll his eyes and look around the room. "You never resigned."

"What? You used to work for them?" Donna was dumbfounded.

"Yeah, a long time ago," he whispered back. "Back in the '70s. Or was it the '80s? But it was all a bit more homespun back then."

"Times have changed, sir," the Colonel butted in.

"Yeah, that's enough of the 'sir'"

"Come on now, Doctor. You've seen it. You've been on board the Valiant. We've got massive funding from the United Nations all in the name of Homeworld Security," Martha explained.

"A modern UNIT for the modern world," the Colonel said proudly.

"What, and that means arresting ordinary factory workers, in the streets in broad daylight? It's more like Guantanamo Bay out there. Donna. By the way, Donna Noble, since you didn't ask. I'll have a salute," she demanded from the Colonel.

He looked to the Doctor to see if he sould take the woman seriously. The Doctor nodded. He followed the order and gave Donna a salute. "Ma'am."

"Thank you," she replied, satisfied.

"So tell me, what's going on in that factory?" The Doctor got down to asking his questions. Colonel Mace didn't hesitate to answer.

"Yesterday, 52 people died in identical circumstances, right across the world, in 11 different time zones." A screen depicting the world map displayed each location. "05:00 in the UK, 06:00 in France, 08:00 in Moscow, 13:00 China…"

"You mean they died simultaneously?"

"Exactly. 52 death at the exact same moment worldwide."

"How did they die?"

"They were all inside their cars," Colonel Mace revealed.

"They were poisoned. I checked the biopsies, no toxins. Whatever it is left the system immediately." Martha added.

The Doctor turned his head in thought. "What have the cars got in common?"

"Completely different makes, but they're all fitted with ATMOS. And that is the ATMOS factory," Martha answered.

"What is ATMOS?"

"Well, come on, even I know that," Donna said. "Everyone's got ATMOS."

Martha motioned for the Doctor and Donna to follow her and Colonel Mace. "It stands for Atmospheric Omission System. The ATMOS in your car reduces CO2 emissions to zero," the said as they walked across the gangway overlooking the factory floor.

"Zero? No carbon, none at all?"

"And you get Sat Nav thrown in, plus 20 quid in shopping vouchers if you introduce a friend. Bargain," Donna added.

"And this is where they make it, Doctor. Shipping worldwide. 17 factories across the globe, but this is the central depot, sending ATMOS to every country on Earth," Colonel Mace explained.

"And you think ATMOS is alien?"

"It's our job to investigate that possibility. Doctor?"

The Colonel continued down the corridor into a room behind the clear plastic drapes. "And here it is, laid bare. ATMOS can be threaded through any and every make of car." He held up the device to show the Doctor.

"You must have checked it before it went on sale."

"We did. We found nothing. That's why I thought we needed an expert," Martha said.

"Really? Who'd you get?" The Doctor asked as he slid on his brainy spectacles. The other three stared at him in response. "Oh, right! Me, yes. Good."

Donna stepped closer to the Doctor to ask questions of her own. "Okay, so why would aliens be so keen on cleaning up our atmosphere?"

"A very good question."

"Maybe they want to help, get rid of pollution and stuff."

"Do you know how many cars there are on planet Earth?" He asked. Before she could answer he told her. "800 million. Imagine that. If you could control them, you'd have 800 million weapons," he whispered to Donna.

After examining the device, the Doctor was able to report his findings. "Ionising nano-membrane carbon dioxide converter, which means that ATMOS works, filters the CO2 at a molecular level."

"We know all that. But what's its origin? Is it alien?" The Colonel questioned further as the Doctor spun around and placed the device on a sample muffler. The Colonel followed.

"No, but it's decades ahead of its time. Look, do you mind? Could you stand back a bit?"

"Sorry. Have I done something wrong?"

"You're carrying a gun. I don't like people with guns hanging around me, all right?"

"If you insist," the Colonel before turning and walking away. Martha gave him an apologetic smile.

"Tetchy," she accused with her arms crossed.

"Well, it's true."

"He's a good man."

"People with guns are usually the enemy in my books. You seem quite at home."

"If anyone got me used to fighting, it's you."

"Oh, right. So it's my fault," he said as he scanned the device with his sonic screwdriver.

"Well, you got me the job. Besides, look at me," she insisted. He looked up at her with his eyebrows raised. "Am I carrying a gun?"

"I suppose not."

"It's all right for you. You can just come and go, but some of us have got to stay behind. So I've got to work from the inside. And by staying inside, maybe I stand a chance of making them better."

"Yeah?" A proud smile crept onto his face. "That's more like Martha Jones."

"I learnt from the best."

"Well…"

"Oi, you lot," Donna yelled as she walked in. "All your storm troopers and your sonics, rubbish. You should've come with me."

"Oh, where have you been?" The Doctor asked.

"Personnel. That's where the weird stuff's happening. In the paperwork. 'Cause I spent years working as a temp. I can find my way around an office blindfold. And the first thing I notice is an empty file," she said as she showed said file.

"Why, what's inside it? Or what's not inside it?"

"Sick days." She opened the binder flat. There was nothing inside to display. "There aren't any. Hundred of people working here and no one's sick. Not one hangover, man flu, sneaky little shopping trip. Nothing. Not ever. They don't get ill." She closed the file for emphasis.

"That can't be right," the Colonel responded.

"You've been checking out buildings, but you should've been checking out the workforce," Donna assessed.

"I can see why he likes you," Martha said with a smile. "You are good."

"Super temp," Donna said with a light chuckle.

"Dr. Jones, set up a medical post. Start examining the workers. I'll get them sent through," the Colonel said as he left.

"Come on, Donna. Give me a hand," Martha said.

The Doctor ran out after the Colonel. "So, this, this ATMOS thing, where did it come from?"

"Luke Rattigan himself."

"That himself would be?" The Colonel escorted the Doctor to a show the boy's file from their database.

"A child genius. Invented the Fountain SIx search engine when he was 12 years old. Millionaire overnight. Now runs the Rattigan Academy, a private school educating students hand-picked from all over the world."

"A hothouse for geniuses. I won't mind going there. I get lonely."

"Do you think I should warn my mum? About the ATMOS in her car," Donna asked as she pulled up a chair to help Martha.

"Better safe than sorry."

"I'll give her a call."

"Donna, do they know where you are? Your family? I mean, that you're travelling with the Doctor?"

"Not really. Although my grandad sort of waved us off. But I didn't have time to explain."

"You just left him behind?"

"Yeah."

"I didn't tell my family. I kept it all so secret. It almost detroyed them."

"In what way?"

"They ended up imprisoned. They were tortured. My mum, my dad, my sister. It wasn't the Doctor's fault, but you need to be careful. 'Cause you know the DOctor. He's wonderful, he's brilliant, but he's like fire. Stand too close and people get burnt."

Angela Harkness awoke to the vibration of her mobile resonating against the table beside her head. Her hand swiped the object from its rest and flipped it open, ready to greet her caller. Before she could utter a word, the voice of her boyfriend, Ross, came through.

"Have they called you in yet?"

"No. Something wrong?"

"Potentially. Better get out of bed, Love."

"Black or business?"

"Black. Gotta go. Stay safe. See you later."

"Later," Angela said before snapping her mobile shut. She looked through her closet for a black outfit to wear. According to Ross, she might have need of her stealth psychic maneuvers.

"You're not coming with me. I want to talk to this Luke Rattigan, not point a gun at him," the Doctor insisted to the Colonel following him.

"It's 10 miles outside London. How are you going to get there?"

"Well, then, get me a jeep."

"According to the records, you travel by TARDIS."

"If there is a danger of hostile aliens, I think it's best to keep a super-duper time machine away from the front lines."

"I see, then you do have weapons, but you choose to keep them hidden. Jenkins."

"Sir," Ross responded immediately.

"You will accompany the Doctor and take orders from him."

"Yeah. I don't do orders," the Doctor commented.

"Any sign of trouble, get Jenkins to declare a Code Red. And good luck, sir."

"I said, 'No salutes'."

"Now you're giving orders."

"Getting a bit cheeky, you are," the Doctor muttered.

"Doctor," the Colonel said as he departed.

Just then, Donna came straight for the Doctor. "Oh, just in time. Come on. Come on, we're going to the country. Fresh air and geniuses. What more could you ask?"

"I'm not coming with you. I've been thinking. I'm sorry. I'm going home."

"Really."

"I've got to."

"Well, if that's what you want. I mean, it's a bit soon," the Doctor said with a brow lowered. Donna just stared at him. "I had so many places I wanted to take you. The Fifteenth Broken Moon of the Medusa Cascade. The lightning skies of Cotter Palluni's world. The Diamond Coral Reefs of Kataa Flo Ko. Thank you. Thank you, Donna Noble. It's been brilliant. You've… You saved my life in so many ways. You're… You're just popping home for a visit, that's what you mean."

"You dumbo," she said in jest to the Doctor.

"And then you're coming back."

"D'you know what you are? A great big outer-space dunce."

"Yeah."

"Ready when you are, sir," Ross interjected from the Jeep.

"What's more, you can give me a lift. Come on. Broken Moon of what?"

"I know. I know."