The Master found life as a cat to be quite enjoyable after he had gotten over his initial shock. The Doctor insisted on feeding him fresh meat and fish and letting him sleep on his bed. Unfortunately for the Master, the Doctor had a habit of using him as a pillow, and drooling over his shiny black fur when he actually bothered to sleep. The Master wriggled out from under the other Time Lord's head and shook himself in disgust, groaning as the three bells on his collars tinkled musically. The Doctor had insisted on him wearing a collar after the first few nights of tripping over the Master as he lounged in the dark hallways. 'Urgh! First it was the drums and now it's the bells!'
It was during one of his nightly prowls that the Master found himself outside Martha's bedroom door. The Master smirked to himself. 'Time for a bit of mischief,' he thought, just before he jumped up at the door handle and managed to open the door. He sneaked in, trying to avoid tinkling loudly, saw Martha fast asleep, and jumped on the bed. Martha twitched slightly, but didn't wake.
The Master leaned forward and used his whiskers to tickle the young doctor's nose and gave a yowl of indignation. "Oh, go away, Master," Martha groaned, not opening her eyes and pulled the quilt over her head, almost knocking the feline time lord off the bed in the process.
The Master's yowls got louder and more urgent and he bumped his head against hers in a surprisingly affectionate gesture. 'Wake up! I want something to eat.'
"Go bother the Doctor," Martha ordered, pretending she didn't hear that snort of derision aimed at her. "He's the one that feeds you, waters you, strokes you..."
'...Drools all over me? I may have fur but I don't have 'pillow' written on me.'