A/N - Welcome back to old readers and new ones!

There was a high-pitched noise and the sound of scuttling feet.

"What the hell was that?" Courtney asked.

"Duke, is that you?" Lundvik asked him.

"I don't sound anything like that."

"Can you try and get the lights back on?"

"That's what I'm doing."

The Doctor grabbed for Lundvik's flashlight. "Torch. Give me your torch. Whatever it is, it's in here."

There were sounds of running claws and screeching as they looked around.

"I think we've found your alien."

A giant black and red spider came towards them from a corridor.

Elise stood frozen in fear, unable to move.

The Doctor and Clara grabbed her and pulled her with them.

"Back, back, back! We need a door. A door, a door!" the Doctor yelled.

"Here! Here!" Clara said.

They ran to the door.

"The door's locked."

"Come on, come on! There's no power to work it. Come on!" the Doctor yelled.

"Doctor."

The Doctor pulled them behind a table. "Stay still. It's sensing movement. It can't see you. Fast movement. There must be another exit through there. Slowly. Slowly. Head to that exit. Slowly. Slowly. Slowly, slowly."

The Doctor took Elise's hand and they slowly made their way over to the corridor. "Gently, gently. When I say run, run."

"Who made you the boss?" Lundvik asked.

"Well, you say run, then."

Duke came out of the corridor.

"Duke!" Lundvik gasped.

The spider jumped on Duke.

"Duke!"

The locked door started opening.

"Run! We have power. Run!" the Doctor told them.

"Quick, it's shutting," Clara said.

The door slammed shut, but Courtney was on the other side. "Miss!"

"Courtney! Courtney!" Clara screamed as Courtney started floating.

"Miss!"

"Courtney! The power's gone again."

"It's killed him. It's coming in here! Doctor, it's coming in here!"

"You'll be okay!" the Doctor assured her, trying to sonic the door.

Lundvik pressed a button on her communicator. "Henry? Henry?"

"Courtney, look at me. Look at me! Courtney!" the Doctor told her.

The spider was starting to walk across the ceiling.

"Get to the wall, get yourself down here!" The Doctor pulled the glass window out of the door. "Courtney, grab my yo-yo!" The Doctor tossed it her and she caught it.

The power came back on and she dropped to the floor.

The spider lunged at her.

Courtney screamed and pulled something out of her backpack.

"Courtney!" the Doctor yelled.

She stood up and sprayed the spider with something.

The spider collapsed on its back and died. The door opened and they ran back into the room.

"Courtney!" Clara gasped.

Courtney held up a spray bottle of cleaner. "Kills ninety nine percent of all known germs."

"And space spiders, apparently," Elise said.

"Good stuff, Courtney. Just don't try that at home, okay?" the Doctor told her.

"You all right?" Clara asked her.

"Why did I just fly? This is nuts."

The Doctor scanned the spider with his screwdriver. "Did you say germs? Oh, God, this is incredible. Look at the size of it. It's the size of a badger."

"It's a giant space spider. Great," Elise said.

"Doctor..." Clara said.

"It's a prokaryotic unicellular life form, with non-chromosomal DNA. Which, as you and me know. Well, not you and me. Well, you, certainly not. You and me, yes, scientists know, this is a germ. You flew because that one point three billion tons shifted. It moved. It's an unstable mass," the Doctor explained.

"I'm scared, Miss," Courtney said.

"Okay," Clara told her.

"He'd just had a grand-daughter. Elina. She was his first. He was my teacher. He taught me how to fly. We were both given the sack on the same day," Lundvik said, mourning for her friend.

"Which way to the Mare Fecunditatis?" the Doctor asked.

"Please can I go home now? I'm really. I'm really sorry, but I'd like to go home," Courtney said.

Clara nodded and looked at the Doctor.

They trekked across the moon's surface, making their way back to the TARDIS.

"Henry, come in. If you don't mind, Henry, come in," Lundvik said.

"Doctor, this is dangerous now," Clara told him.

"It was dangerous before. Everything's dangerous if you want it to be. Eating chips is dangerous. Crossing the road. It's no way to live your life. Tell her. You're supposed to be teaching her."

"Look, I have a duty of care, okay? You know what that is?"

"Course I know what a duty of care is. What are you suggesting? She's fine. What are you, thirty-five?"

Elise rolled her eyes. Even after all this time, he had trouble with human ages.

"Fifteen," Courtney corrected him.

They finally reached the TARDIS and Courtney climbed aboard.

"Now, don't touch anything. Especially Elise's books," the Doctor told her.

"You got any games?"

"Oh, don't be so stupid!"

"Can I get reception...?"

"Get in!" the Doctor slammed the TARDIS door.

"Why are you shutting her in? We don't need to stay, do we?" Clara asked.

"Eh?"

"It's obvious, isn't it? The moon doesn't break up."

"How do you know?"

"Because I've been in the future, and the moon is still there. I think. You know the moon is still there, right?"

"Maybe it isn't the moon. Maybe it's a hologram or a big painting, or a special effect. Maybe it's a completely different moon."

"But you would know."

"I would?"

"If the moon fell to bits in 2049, somebody would've mentioned it. It would have come up in conversation. So it doesn't break up. So the world doesn't end. So, let's just get in the TARDIS and go."

"Clara, there are some moments in time that I simply can't see. Little eye-blinks. They don't look the same as other things. They're not clear. They're fuzzy, they're grey. Little moments in which big things are decided. And this is one of them. Just now, I can't tell what happens to the moon, because whatever happens to the moon hasn't been decided yet. And it's going to be decided here and now. Which very much sounds as though it's up to us."

"Clara, the future can always change. Only certain things are fixed points, everything else is in flux," Elise told her.

"What Elise said."

"None of you are going anywhere. I've lost my crew. We were the last astronauts. This is the last shuttle, these are the last nuclear bombs. We're the last chance for Earth, and you're staying to help me," Lundvik said.

"Decision made," the Doctor said.

"Yeah," Clara agreed reluctantly.

They left the cargo hold and walked until they survey equipment.

"What is killing the moon?" the Doctor asked.

"How can the moon die, though?" Clara countered.

"Everything does, sooner or later."

"Can we save it?" Lundvik asked.

"Depends what's killing it."

Lundvik glanced over the cliff edge and saw three spacesuits covered in cobwebs. "There are the other three."

They went down to investigate.

"Is it those germ things, then? Are they like cockroaches? Is it, is it an infestation?" Clara asked.

"Is it?" Lundvik asked.

"Well, I've only seen one of them. It would take an awful lot more to cause the moon to put on one point three billion tons."

A giant spider came out of a cave and jumped on the Doctor.

"Doctor!" Clara yelled. Clara tried the spray.

"It's a vacuum. It won't work," Lundvik told her.

The three of them grabbed at its legs and pulled it off the Doctor.

It crawled back into the cave.

"Well, that makes two," the Doctor said.

"Sunlight," Clara said.

"Sunlight?" Lundvik asked.

"If they're germs. My nan says it's the best disinfectant there is."

"Shine your light down there," the Doctor told her.

Lundvik did and revealed several spiders. "Where have they come from?"

"Maybe they've been there all the time. It's warmish. They're multiplying, feeding, evolving," the Doctor said.

They quickly left the survey spot.

"Doctor, if the moon breaks up, it'll kill us all in about forty five minutes," Lundvik told them.

"I agree. Unless something else is going on," the Doctor said. He used his yo-yo to get a sample from a crack and it came back wet.

"There's no water on the moon," Lundvik said.

"It's not water. It's amniotic fluid. The stuff that life comes from. I've got to go down there," the Doctor said.

"Doctor."

"Back to your shuttle. Get your bombs ready." He turned to Clara and Elise. "You, get to the TARDIS. Get safe. Get Courtney safe. I will be back." He took the spray bottle from Clara.

"What? No. Doctor. Doctor!" she yelled.

The Doctor jumped down the crack.

"Doctor!" Clara yelled.

"I hate it when he does that," Elise muttered.

"Will he?" Lundvik asked.

Clara threw her arms up in the air before sighing.

"Will he be back?"

"If he says so, I suppose he will."