Thirteen ran to the TARDIS, hearts pounding as she slammed the doors behind her. She threw herself at the console, hoping that she had enough time. One regeneration was bad enough - she didn't want it to happen again so soon. Especially with her former self running around on Earth. She pulled the leaver as the ancient machine wheezed into action.

"Come on girl." She gasped as the TARDIS whirred and wheezed. The column spun and glowed gold as it dematerialised into the ether. "Thank you." She paused. "Get me back to the others. Please, just get me back." She gasped as she rested her head back against the console.

xxxxxxxxxxxx

"Right then." Graham followed Ace along the corridor. He had no idea what had just happened but he couldn't help the guilt that washed over him. Sighing he tried to reason with himself that Grace would understand. That she wouldn't want him to be on his own. He knew if roles had been reversed he would want her to find someone she liked, that she could be happy with. Someone who would make her smile again. It didn't work, the feeling he had been cheating on her laid heavy in his stomach. He looked up as Ace called his name.

"Yeah?"

"You ok?"

"Me? Er."

"Grace? Is that it?" Ace watched as he nodded, looking away. "I get it. I really do. You love her. You will always love her and you think kissing me, whatever you think of me is betraying her."

"Sorry. I."

"You've nothing. Nothing to be sorry for. I get it. I really do." She smiled, thinking of her former partner, now long gone. "If you want to chalk this up to a mistake, if you want to leave this."

"No, no I aint saying that." He grabbed her hand. "I really ain't, Ace. Look."

"Ok, then." She smiled. "How about we go and help the Professor and then you can buy us coffee? No pressure." She raised an eyebrow as Graham nodded.

"Bleedin' 'ell the Doc! She's going to have a fit if we aren't there when she gets 'ome."

Ace laughed, leading him back into the main office, ignoring the pointed looks from Tegan and Dan.

xxxxxxxxxxxxxxxxxxx

"Rightio!" Ten pushed his heavy framed glassed back up his nose. "What do we know about Sontarans?"

"Mr Potato Head with a Napoleon complex." Dan replied. "They don't like frying pans."

Ten laughed. "Saw a Sontaran with a Napoleonic uniform on before. Bit too like the real thing if you ask me. Mind you, he didn't 'alf go on. Talk about talk a glass eye to sleep. France this, France that."

"ABBA wrote a song about him." Yaz shrugged.

"ABBA wrote about a Sontaran?"

"No, about Napoleon. I think. It was called Waterloo." She shrugged as Graham and Dan shook their heads slightly "Sorry, Sontarans. Like Dan said. Warriors, from Sontar."

"Yes but they keep trying to destroy Earth. Met them in the Crimea." Dan answered. "They were there in the same War as Mary Seacole. Now there was a lady not to be messed with. People go on about Florence Nightingale but Mrs Seacole, what a top girl."

"Ok can we focus?" Kate folded her arms as she looked at the satellite map on the screen. "The Sontarans have released something. A gas?"

"WHAT? WHAT!" Ten jumped across the room so he could see the screen. "Well, well. They are serious. That is seriously not good. I mean on a scale of one to ten of badness. That is a twelve."

"Right, well. What is it?"

"Sontarans, Kate. They are not the type of species to just toddle off into the night. They have done this before. Karton 5. Sat there for ages, just watching the planet and then they release this blanket. It smothered all life. Deprived the planet of Carbon Dioxide."

"We need oxygen." Ian raised an eyebrow. "Doctor."

"I know, yeah but we need air. You lot do anyway and air is comprised of more than just oxygen. Its a balance. If you remove one thing the other can't be produced. Look at trees."

"Trees?" Tegan raised an eyebrow.

"The lungs of the planet." Graham folded his arms. "They make oxygen. But they need carbon dioxide. They 'breathe' that in and breathe 'oxygen' out. If they kill the trees then we all die."

"He's right." The room fell silent as Ten rocked back on his heels just as the sound of the TARDIS materialising could be heard.