Rose is sitting by his bedside, young-faced and bright-eyed, holding his wrinkled hand. His breathing is labored, his single heart is giving out.
"Rose Tyler, I love you," he rasps.
"Always," Rose replies, leaning over his frail form. She presses a kiss to his forehead. Then a kiss to his cheek. With eyes open, she presses a final kiss to his lips — lips she’s kissed thousands of times.
In that moment, something sparks hot in her chest. Golden fire shoots through her veins, sparks over every nerve, pooling behind her eyes with such infinite pressure she feels like she might burst. The golden fire flows out of her, drifting through the air in a glowing stream, directly into the Doctor’s eyes.
It’s terrifying and for a heart-stopping moment she’s back on Satellite Five, wrapped up in leather-jacketed arms, staring into blue eyes made yellow by vortex energy. Now those arms are old and frail and in a dressing gown, and the eyes are brown and staring back at her in shock.
The golden fire fades, and the Doctor is still lying in bed, but he isn’t a frail old man anymore. He’s regenerated — they’ve regenerated, together. Same brown hair, brown eyes, freckled cheeks, but new young skin, new young everything.
"Well," he says, gripping her arm. "That was unexpected."