The Master paced back and forth across the room, muttering to himself while Cara worked on the final calibrations of the machine. Cara cast a wary eye in his direction and he stopped.
"Well?" he demanded, "what is it? Is it ready?" His beady dark eyes glimmered intensely. Cara wondered how his face always managed to appear to be lit from below. She took more time than was necessary before answering him. The build of anticipation in his face was worth the glare he sent her.
"Well, is it?" he exploded.
Cara regarded him coolly, "Don't rush me. We'll only have one chance at this."
"Get on it then!" the Master growled, smacking his hand somewhat absently with his cane.
Cara sent him a glare just long enough to demonstrate that she did not appreciate being ordered around, "Too bad you skipped class the decade we were finishing the design on this, it would have gone quicker with two."
The Master returned her glare and turned back to pacing.
The tree exterior shook, rattling the laboratory. Neither Time Lord paid it any attention. The end was here at last, and they were prepared.
Working efficiently, Cara entered the last protocols into the machine and stood back.
"Is it ready now?" the Master stared at her with wide-open, desperate eyes. He felt it too, the wall that was approaching. He knew, as Cara did, that time had run out for the Time Lords.
Cara stepped back again, knowing the Master could barely contain his excitement. She was right. He dove for the controls, pausing to savor the moment, but only just. With a flourish, he jammed his hand down onto the ignition key and flipped it. The two watched, satisfied, as the box began to emit a nearly subsonic pulsing hum as well as a strobing white light.
The Master smiled, leaning against the box, celebrating his victory. His entire body tensed up and the smile left his face as the box's pulses sped up, beyond the range where they were distinguishable from a solid buzz. The Master tried to pull away from the machine, but he found that his hand was fuzed to the ignition key.
"What did you do to it?" he hissed at Cara as she watched him writhe stiffly.
"What it was always intended to do," Cara replied quietly, staring at him with an icy expression, "how does it feel to be so full of power, Master?"
"Don't mock me, you child!" he gasped, trying to lock his gaze on her. She felt his bewildered mind sending flailing probes, bumping into hers now and then but not able to connect. Again and again, he tried to retaliate for the situation that he felt Cara had put him in. But she had given up her focus on him and now turned inward, strengthening her mental barrier against the onslaught the machine was pumping into the room.
The bark door swung open, breaking the concentration of both Cara and the Master. The Doctor stood in the doorway, evaluating the situation with a glance. He tried to step into the room, but was met with invisible resistance.
Cara saw the Doctor trying to get in and called to him, "It's going to overload, get out of here!"
"You too!" the Doctor yelled back.
Cara looked at the Master, who was wrestling with the box to get his hand free. She moved toward the Master, "Come on!"
He gave her a seedy squint and tore at the machine with his fingernails, "Can't you see I'm trying, you fool!"
Cara reached to the ornamental dagger hanging at her side and took it from its sheath. She approached the Master, "Let me help you."
"You! You did this to me!" the Master eyed the blade, "I don't need you carving me up. For what, to save me? You want to kill me!"
"I want to destroy the machine," Cara growled, "killing you was never the plan."
"I don't want your help at any rate!" the Master stopped struggling to push Cara back with his mind. She pushed back and began to hammer the machine with the dagger.
"Cara! Master!" the Doctor cried, powerless to help behind the field of concentrated energy, "It's getting crowded out here! Come on!"
Cara knew this. She could hear the sounds of the battle raging, the screams and blasts of her sworn enemy. Inside the lab, her not-so-sworn enemy wasn't being much quieter. He howled and clawed and fought as the machine's hum rose to a high, squealing pitch. The light got even brighter, becoming a blinding flare in the center of the room. Cara grabbed the Master and pulled in an attempt to free him.
"Let go of me!" the Master threw his considerable weight roughly and shoved Cara at the same time. She crashed sideways, colliding with the damaged machine controls. Shock showed in her eyes as she pushed away from the machine. She looked down, finding that the side of her clothing that had connected with the controls was dark and red and wet. She stared at it with fascination, as if she had suddenly realized something amusing.
The Master was too busy examining his newly-freed hand to notice at first. The next thing he noticed was the Doctor's anguished expression. And only then did the Master turn to see what had become of Cara. Her eyes met the Doctor's. He nodded sadly and banged the doorway frame with his fist. The Master backed away from both of them, having noticed that his freed hand was missing its fingertips. The fingers seemed to dissolve into the air. His mouth opened in a silent scream and he slid to the floor.
Cara held her hands in front of her, looking at them dully. Between the patches of drying blood, her skin sparked and shimmered with sky-orange flame. Her mouth opened slightly, but she couldn't think of anything to say. Beside her, the brilliantly white flaring box threw showers of white and blue pinpoints everywhere, the squeal turning to a brief roar. Then it went completely silent as Cara stood transfixed. The light redirected and shot upward.
The second that the machine channeled the energy out, the Doctor dashed into the room. He grabbed Cara and helped her quickly to the door.
"Hold on," he whispered to her, "we have to get you away from this machine before it overloads or you'll forget all over again."
"Forget…" Cara mumbled, "I wish I could."
The Doctor brushed a stray lock of hair out of her face, "I'm sorry." Together, they stumbled out of the tree lab and back toward the Doctor's TARDIS.
The Master didn't even notice them leave. He did, however, take a metal object out of his pocket, even as his entire arm was vanishing.
"I'll be back, Doctor," the Master muttered without a shred of sanity, "I'll be in the last place you look."
He took a final breath, closed his eyes, and opened the fob watch with his one remaining hand as his other arm vanished completely.