The Beginning Of Forever 2/3 or 4
Shaun smiled at Sarah Jane. "You understand what's going on."
"Somewhat," Sarah Jane said. "Donna's Gallifreyan?"
"She didn't start out that way. She was human." Shaun smiled. "But then there was a two-way Time-Lord-Human biological metacrisis."
"I remember that," Sarah Jane nodded. "I was there. I saw the second Doctor."
"It wasn't just my mind," Donna suddenly realised. "It was altering my body, too, wasn't it? That's why I had the fever..."
"He didn't realise that, though," Shaun said. "He thought it was all your mind. He thought he was protecting you."
Donna looked at her open ring. "Amazing," she breathed. "All the energy went into this until my body had changed enough...."
"That it was safe for you to remember," Shaun nodded. "And it had slowly changed far enough that after the 'defence mechanism' kicked in it could finish without the fever and the pain."
Donna met his eyes. "Wait - how can you possibly know all this?"
His smile vanished. "Shaun Temple never existed, Donna. I'm a clone - created to help you remember and adjust. To help you survive. To be with you until you find the Doctor again."
"A clone?" Donna gulped. "Who cloned you?"
"The Ood," came the surprising reply. "This song of his may be ending soon - yours was never to end. You were always to be by his side, through all of his songs. And now - you can be."
Her hand moved to cover the second heart again. Her voice was filled with wonder. "You mean....I-I can regenerate?"
"If you can," Sarah Jane's voice was full of that same wonder. "If you can, why, then, you could potentially be with him for centuries!"
Donna broke into a delighted grin at that thought. Then she sobered. "But - But he could be anywhere! Any time! I - He thinks that I'm.... He'll not come back for me."
"Oh, he'll come back." Sarah Jane turned and mounted a set of stairs. "I'll see to that. Mister Smith! I need you!"
Donna and Shaun blinked at each other and then dashed after her.
On to the conclusion!