I don't own Doctor Who.
"Don't worry, I'll keep a close eye on her," the Doctor promised, following Donna out of her grandparents' house and onto the sidewalk. Groups of yelling children rushed past carrying bags of candy, their families trying to catch them. Chuckling, the Time Lord led his companion in the direction of the TARDIS, barely dodging an eager boy while ignoring the woman's snort. Rounding a corner, they came in sight of the time machine with a crowd of onlookers surrounding it, conversing in hushed voices. As the Doctor shouldered through the mass, the police box suddenly morphed into a stop sign, much to the Time Lord's surprise. "Oi, for the last time, the chameleon circuit doesn't work, so quit fooling around!" With a sharp ping, the TARDIS returned to its original shape, its light flashing once. Nodding, the Doctor inserted his key, aware of the dozens of stares on his back. "That's better, old girl."
"Hey, she's just tryin' to have a bit of fun!" Donna barked, satisfied when he jumped, shooting her a glare. Dismissing his anger with a wave of her hand, she yelped as all the lights in the TARDIS abruptly went out, leaving the moon to illuminate the ship's interior. "Uh, what's going on? Doctor!" Positioning herself in the doorway, the woman refused to take another step, watching the Time Lord walk forward, scanning the darkened console room. "You going to answer me, or-"
Motioning for silence, the Doctor whipped his head to the side, sure he had seen a black shadow dark across the far wall. "Is there anybody in here? Hello?" A loud bang, followed quickly in session by three more, made him start violently, hearts pounding. The noise repeated, shaking the TARDIS itself-and jogged the Time Lord's memory. "No, no, no! Donna, stay out..." Whirling as his companion screamed, he saw her thrown to the ground by an unseen force, the time machine's door snapping shut. Bellowing the woman's name, began to hurry for the exit, but was slammed painfully to the floor, pressure on his chest. Something cold caressed his face, slipping towards his ear, the faint drumbeat he knew and feared playing in his head. "G-get away from me! You should know that there is no way you're going to-" Sentence becoming a howl of agony, the Doctor felt his mind succumb to the probing touch of the ghostly entity, falling into unconsciousness.
Outside, Donna beat frantically on the side of the TARDIS, calling for the Doctor in a high-pitched voice. Just when she raised her hand to pound one more time, the door creaked open, revealing the Time Lord. "Thank God, I thought-Doctor, are you ok? Why are you staring at me like that?" Swallowing with difficulty, the woman took a cautious step forward, pausing as he advanced, a predatory look in his brown eyes. Even the group of people around them sense danger, for they all retreated to what they deemed a safe distance.
"I'm not the Doctor, Donna Noble, I am the Master." Baring his teeth, the possessed Doctor lunged, his grasping fingers brushing against her sleeves. Pursuing Donna's fleeing form, he almost caught her, but she ducked into an alley. "Embrace your inevitable death, woman! Let the Doctor see what he is capable of with his own hands!" Skidding to a halt, the Time Lord found the woman huddled between some garbage cans. About to take a step, he suddenly collapsed onto his knees, squeezing his head. "I will not let you interfere in my plans, Doctor!"
Warily creeping toward the fallen Doctor, Donna knelt next to him, tilting his chin to peer into his eyes, finding intense pain in them. "Fight him, Doctor, I know you can." Leaning down, she brought her lips to his, drawing back a second later to witness a stream of grey leave his body and waft into the night air.
Gaining control of his body, the Time Lord groaned and lifted his head. "Let's get out of here before the police come." Allowing his companion to hoist him to his feet, they both made their way back to the TARDIS. However, Donna failed to notice the brief glow in the Doctor's gaze as he locked the ship's door.
Just something I thought of.