Outside the industrial estate, people were putting on their gas masks as the foggy smoke was getting thicker. Mace showed one of the new guns to the Doctor. “Latest firing stock. What do you think, Doctor?”
The Doctor looked down at Lilith. “Are you my mummy?”
She cracked up. “You’ve been waiting to use that line since you got the mask, haven’t you?”
“Should I send you to your room?”
“If you two could concentrate,” Mace snapped. “Bullets with a rad-steel coating. No copper surface. Should overcome the cordolaine signal.”
“But the Sontarans have got lasers,” the Doctor pointed out. “You can't even see in this fog. The night vision doesn't work.”
“Thank you, Doctor. Thank you for your lack of faith. But this time, I'm not listening.”
Lilith snorted. “You got a handful of men killed because you didn’t listen last time.”
Mace removed his gas mask and gave some speech to the men that Lilith assumed was supposed to be inspiring, but it just made her scoff. There was a sudden massive downdraft, blowing the gas away. “It's working. The area's clearing. Engines to maximum.”
“It's the Valiant!” Lilith shouted over the noise, taking of her mask.
“UNIT Carrier Ship Valiant reporting for duty, Collector. With engines strong enough to clear away the fog.”
The Doctor took off his gas mask. “Whoa, that's brilliant.”
Mace raised an eyebrow. “Getting a taste for it, Doctor?”
“No, not at all. Not me.”
“Valiant, fire at will,” the Colonel ordered. Six green beams from the Valiant converged to form one that hit the ATMOS factory, reminding Lilith painfully of Torchwood doing something similar to the Sycorax ship.
She shook the thought out of her head as everyone rushed back into the factory. The Colonel went one way, but the Doctor and Lilith went the other.
“Shouldn't we follow the Colonel?” Martha asked.
The Doctor scanned the area with the sonic screwdriver. “Nah, it’s the three of us, Martha Jones. Just like old times. Alien technology, this-a way.”
When they made it to the basement, the lights in the hallway turned on. “No Sontarans,” Lilith noted. “They can't resist a battle. Here we go.” She pushed open the door at the end of the hallway to reveal some sort of laboratory with the real Martha Jones lying strapped to a platform. “Aunt Martha!”
The Doctor ran over to check on her. “Oh, Martha, I'm so sorry. Still alive.”
The clone pointed a gun at the Doctor's head. Lilith whipped out her blaster and pointed it at the clone.
“Am I supposed to be impressed?” the Doctor scoffed.
“I've been stopping the nuclear launch all this time,” the clone said in a bragging tone.
“Doing exactly what I wanted. I needed to stop the missiles, just as much as the Sontarans,” the Doctor said. “I'm not having Earth start an interstellar war. You're a triple agent.”
“When did you know?”
“About you? Oh, right from the start. Reduced iris contraction, slight thinning of the hair follicles on the left temple.”
"Blinking less often,” Lilith added, “and honestly, you stink. You might as well have worn a T-shirt saying clone. Not in front of Uncle Jack, though. That would be a bad idea.”
“You remember him, don't you?” the Doctor questioned. “Because you've got all her memories. That's why the Sontarans had to protect her, to keep you inside UNIT. Martha Jones is keeping you alive.” He took the device off Martha's head, and the clone collapsed. Lilith kicked the gun away. Martha gasped for breath as the Doctor comforted her. “It's all right, it's all right, I'm here, I'm here. I've got you, I've got you.”
“There was this thing, Doctor, this alien, with this head.”
“Potato head aliens.” Lilith put her blaster away. “Tell me about it."
Lilith’s phone rang. “Oh! Blimey, I'm busy.” The Doctor answered. “Got it?”
“Yes,” Donna said on the other side. “Now hurry up.”
“Take off the covering,” the Doctor instructed. “All the blue switches inside, flick them up like a fuse box, and that should get the teleport working.”
Martha noticed the clone for the first time. “Oh my God,” she breathed. “That's me.” She went over to the clone.
“Don't touch me,” the copy said.
“It's not my fault. The Sontarans created you, but you had all my memories.”
“You've got a brother,” the clone forced out the words, “sister, mother and father.”
“If you don't help me, they're going to die.”
“You love them.”
The real Martha nodded vigorously. “Yes. Remember that?”
“The gas. Tell us about the gas,” the Doctor insisted.
The clone shook her head. “He's the enemy.”
“Then tell me,” real Martha said. “It's not just poison, what's it for? Martha, please.”
Clone Martha hesitated. “Caesofine concentrate. It's one part of Bosteen, two parts Probic five.”
“Clonefeed,” Lilith realized. “It's clonefeed!”
“What's clonefeed?” Martha asked.
“Like amniotic fluid for Sontarans. That's why they're not invading. They're converting the atmosphere, changing the planet into a clone world,” the Doctor explained. “Earth becomes a great big hatchery. Because the Sontarans are clones, that's how they reproduce. Give them a planet this big; they'll create billions of new soldiers. The gas isn't poison; it's food!”
“My heart. It's getting slower,” the clone managed.
“There's nothing I can do.” Martha said, mournfully.
“In your mind, you've got so many plans. There's so much that you want to do.”
“And I will. Never do tomorrow what you can do today, my mum says, because…”
“Because you never know how long you've got,” the clone finished. “Martha Jones. All that life…” She died.
“Doctor. Blue switches done, but they've found me,” Donna’s voice said from the speakerphone.
“Now!” the Doctor shouted as he soniced the teleport.
Donna appeared. “Have I ever told you how much I hate you?” She hugged him.
“Hold on, hold on! Got to bring the TARDIS down.” He soniced the teleport controls again. “Right, now. Martha, you coming?”
“What about this nuclear launch thing?” Martha asked, holding up her phone.
“Just keep pressing N. We want to keep those missiles on the ground.”
Donna’s eyes flickered from Martha to the clone on the floor. “There's two of them.”
The Doctor glanced up from the controls. “Yeah, long story. Here we go. The old team, back together. Well, the new team.”
The four of them stood in the teleport pod. “We're not going back on that ship!” Donna insisted.
"No, no, no. No. I needed to get the teleport working so that we could get to—” In a flash of light, they appeared somewhere else. “Here. The Rattigan Academy.”
Luke pointed a gun at them. “Don't tell anyone what I did. It wasn't my fault, the Sontarans lied to me, they—”
The Doctor snatched the gun and threw it away. "If I see one more gun…” he muttered.
“You know, that coat sort of works,” Donna commented to Martha.
Martha looked down at herself. “I feel like a kid in my dad's clothes.”
“Oh well, if you're calling him dad, you're definitely getting over him.”
“Oh, no. I’ve already got two sisters,” Lilith said. “And one likes to blow things up.”
They followed the Doctor into the lab where he was building a device. “That's why the Sontarans had to stop the missiles. They were holding back. Because caesofine gas is volatile, that's why they had to use you to stop the nuclear attack. Ground to air engagement could spark off the whole thing.”
“What, like set fire to the atmosphere?” Martha questioned.
“Yeah. They need all the gas intact to breed their clone army. And all the time we had Luke here in his dream factory.” He turned to Luke. “Planning a little trip, were we?”
“They promised me a new world.”
“You were building equipment, ready to terraform El Mondo Luko so that humans could live there and breathe the air with this. An atmospheric converter.” Finishing the device, the Doctor raced outside to the grounds of the Rattigan Academy.
Donna looked out at the smoke. “That's London. You can't even see it. My family's in there.”
“Doctor, hold on. You said the atmosphere would ignite,” Martha said.
“Yeah, I did, didn't I?” The Doctor activated the atmospheric converter, and an energy pulse zoomed up into the grungy clouds. A giant fireball spread in the upper atmosphere. “Please, please, please, please, please, please, please.” The flames raced around the world, consuming the gas without damaging any buildings, and leaving a clear blue sky behind.
“He's a genius,” Luke marveled.
“Just brilliant.” Martha grinned.
“Now we're in trouble.” The Doctor grabbed the atmospheric converter and ran back into the room with the teleport.
‘Dad, what are you doing?’
‘Not now, Lilith.’ And with that, he promptly shut her out. He carried the atmospheric converter into the teleport. “Right. So, Donna, thank you for everything. Martha, you too. Oh, so many times. Lilith, you were amazing, always amazing. Luke, do something clever with your life.”
“You're saying goodbye,” Donna realized.
“Sontarans are never defeated. They'll be getting ready for war. And, well, you know, I've recalibrated this for Sontaran air, so…”
“You're going to ignite them,” Martha finished.
“You'll kill yourself,” Donna protested.
Lilith growled. “Over my dead body.” She flicked a switch on her blaster and shot the Doctor. He crumpled to the ground.
“You’ve killed him!” shouted Donna.
The Time Lady shoved the Doctor’s limp body of out the teleport pod. “No, I’ve stunned him. If anyone’s dying today, it’s going to be me.”
“Just send that thing up on it's own,” Martha suggested, “I don't know. Put it on a delay.”
Lilith shook her head. “I can't.”
“Dad would give them I choice, so that’s what I’ve got to do.” She teleported onto the Sontaran ship. “General Staal,” Lilith said in a commanding voice, “you know what this is. But you’ve got another option. You can go. Pick up and leave. Sontaran High Command won’t need to know what happened.”
“Your stratagem would be wise if Sontarans feared death, but we do not. At arms,” Staal ordered. All the Sontarans aimed their weapons at Lilith.
“I'll do it, Staal,” she warned. “If it saves the Earth, I'll do it.”
“A warrior doesn't talk, they act.”
Lilith gritted her teeth. “I am giving this one chance to leave. I'm warning you.”
“And I salute you. Take aim!”
“Even if you shoot me, I'm still going to press this. You'll die, Staal, you all will.”
“Knowing that you die too. For the glory of Sontar. Sontar-ha! Sontar-ha! Sontar-ha! Sontar-ha!”
All of the Sontarans continued the chant. Lilith tensed, ready to activated the converter, preparing for death.
Suddenly, she was back in the Academy. “What the hell?”
The Doctor dragged her out of the teleport and pulled her into a hug. “Never do that again. Don’t you dare ever do something like that again.”
She hugged him back. “Then stop trying to get yourself killed, Dad.”
Lilith was sitting at the table in the Noble house with Donna and Wilf.
“The streets are half empty. People still aren't driving. There's kids on bikes all over the place. It's wonderful,” Sylvia said. “Unpack that lot, I'm going to see if Suzette's all right.” She left the room
Wilf watched her go. “I won't tell her. Best not. Just keep it as our little secret, eh?”
“Yeah,” Donna admitted.
“And you go with them, Lilith and that wonderful Doctor. You go and see the stars, and then bring a bit of them back for your old Gramps.”
Donna smiled. “Love you.” She kissed him on the cheek.
“It was nice meeting you, Wilf.” Lilith waved as she and Donna walked out the door towards the TARDIS. “You know,” Lilith said, conversationally, “Dad becomes friends with a Sontaran in the future.”
Donna gaped at her. “He what?”
“Don’t worry, he’s a good one. Well, I say good but…” She trailed off, frowning. “You know, I’m not really sure how to describe Strax.”
The Doctor and Martha were already in the TARDIS. “How were they?” Martha asked.
Donna wiped away a tear. “Oh, same old stuff. They're fine. So, you going to come with us? We're not exactly short of space.”
Martha shook her head. “Oh, I have missed all this, but you know. I'm good here, back at home. And I'm better for having been away. Besides, someone needs me. Never mind the universe, I've got a great big world of my own now.”
The door slammed shut on its own and the time rotor activated, throwing everyone around.
“Doctor, don't you dare!” Martha shouted.
“No, no, no. I didn't touch anything.” He pulled the monitor over. “We're in flight. It's not me!”
“Where are we going?” Donna yelled.
“I don't know. It's out of control! Lilith!”
Lilith desperately messed with the console. “She’s flying herself!”
“Doctor, Lilith, just listen to me. You take me home. Take me home right now!” Martha demanded.
Lilith swore in Gallifreyan.