"Third time's the charm."
It was an Earth saying, but the Time Lord said it with conviction, wishing it to be the case. He looked down at Donna's bed where she was sleeping peacefully, no idea that she'd almost died.
The first time he'd wiped her mind, she'd begged to keep her memories, her essence, and he'd ignored her pleas, hoping to save her life by erasing the only time she'd really lived.
The second time he invaded her head, she had run into him on the street, recognized his face and screamed. He vowed not to return to London during her timeline until he regenerated.
This time was different. His brilliant Donna had been ill, desperately so, and though there was no cure on Earth, he knew of a planet with the ability to help her. His hearts had almost stopped when Wilf called, afraid that something had reminded her again, and he was unable to speak for several minutes at the news that Donna was dying, that he needed to come say goodbye. He'd gone to her instantly and saw her lying on her bed, fighting to survive against the poison coursing through her body; his own breath caught in his throat. He shouldn't have left her the candies from Hallifrax, but how was he to know she'd be allergic?
Wilf and the Doctor carried her to the TARDIS (Sylvia refused to have any part of it, blaming him for her daughter's predicament in the first place) and kept her from waking until they arrived at their destination. Her granddad watched over her in the med bay as the Doctor flew them to the small planet, fetching the antidote. It wasn't the easiest to find, but in less than an hour he returned, prepared to help his friend. He walked back into the ship and was met by a sobbing Wilf. He knew what was wrong right away. Donna was dead; she had passed away while he was on the surface.
It couldn't be, he wouldn't let it, and he ran to the sick bay unwilling to accept that Donna was gone, but as he entered the med bay the diagnosis was confirmed: Donna was no longer breathing, nor could he find a pulse. He placed a cool hand to her cheek, feeling the warmth Donna had always shown him when they travelled together. Her skin was pale but still warm; she'd only been dead a couple minutes.
He reached below her and pulled out a medical kit, determined to save her. There had to be a way, even though there wasn't much time; Donna had never given up on him, and he wasn't about to give up on her. When he injected her with the medication he heard her take a sharp breath; she had gone into anaphylactic shock, but the drugs reversed the effects almost instantly. The color slowly returned to her cheeks and her respirations were no longer as pained, her blood pressure raising back to an almost normal level. She was alive.
They returned her to her bed and he couldn't resist sitting with her for a moment, if only to assure himself she was all right. She awoke, her eyes fixing on him. "Doctor," she whispered. "Doctor, am I still sick?" He took her hand, rubbing it gently with his fingers. "No, Donna you're fine," he answered. "I'll always be here to take care of you, okay?" She nodded, smiling at him. "I know. Just can't leave me alone, can you?" she teased. He grinned, checking her pulse again. "Yup, that's me. I'm never leaving," he added sincerely.
"But I can't know it, can I?" Donna asked sadly, a tear escaping the corner of her eye. He shook his head, brushing the wetness from her cheek. "I'm sorry. I'll never forget you, I promise. You'll be okay." Donna squeezed his hand, then released it and allowed him to frame her face with his long fingers. "Goodbye, Doctor," she murmured, "and thank you."
Her eyes closed again as she forgot the last several hours, and as Donna fell asleep she felt him press a tender kiss to her forehead. "Anything for you, Donna," he promised her, then stood and watched her for a few more moments before turning and leaving her to rest.
He wouldn't stop seeing her, but he desperately hoped she would never see him again.