It took a few nanoseconds for it to register in the Master's mind: that sound, like tissue-paper tearing, was the Doctor's breathing.
The Master hadn't been confined to the TARDIS for long, though it already seemed an eternity to him… perhaps to them both. Their alliance wasn't yet worthy of the name, their coexistence a long sullen silence interspersed with violent rows that served to release tension as much as (not that either would yet admit it) an excuse to talk to one another again.
The Doctor hadn't spoken of, nor shown, any physical infirmity in all the time since the Master had first come online in this new robot body. But then the two of them had never properly discussed the events in which the Master had lost his last biological form, and in which the Doctor had lost a companion, much of their confidence, and their joie de vivre: there hadn’t been, emotionally speaking, an opportunity for either of them to open up.
Several hours ago they had arrived in what the Doctor claimed was normally a lovely region of an interesting planet, but this time the TARDIS spatiotemporal controls, programmed remotely of course, had landed the ship near the end of the great brush fires of 3074. Even in the major city where they’d landed, there had been so much smoke that the Master, looking out the doorway as the Doctor left, couldn't even enjoy watching an unimportant place burn. There were scattered memories of conflagrations much closer to home, but these he quickly suppressed.
Mostly, he was glad he didn't need to breathe anymore. Best never to mention the things he appreciated about the mechanical body, though, lest acceptance of his fate be confused for gratitude.
So the bad air must have been part of the problem. And then, while the Doctor was still out, the rains had started. The Master realised, with some irritation at himself for even thinking of it, that the Doctor would be terribly relieved that the locals were out of danger. Even more disturbing to him was his own feeling of relief at the Doctor being out of danger.
Indeed, having finished whatever petty errand the Time Lords had sent them here for, the Doctor eventually returned to the TARDIS, pale and shivering, wrapped tightly in their voluminous cape and coughing through their scarf. They'd been back just long enough to drop the waterlogged cape in an untidy heap on the floor and set the ship to return to the vortex, when the Master came in from a side corridor. As the noise of dematerialising faded, he recognised the other more painful sound.
“Doctor...” The Master tried to affect idle curiosity.
“You’ve… come to… gloat?” they managed between shallow breaths.
The Master almost didn't dignify this with a response; accurate or not, it didn't seem sporting to argue with someone who was fighting for enough air to get words out.
The Doctor was leaning heavily on the console, both hands gripping the time-polished wood trim at the edge. Those startling blue eyes stared unseeing at the various dials, their owner's mind elsewhere in anxious concentration. The Doctor was breathing much too fast, wheezing loudly with each exhale.
The Master took in just how ill the Doctor was, how helpless they'd be if he chose to push them aside, or worse. He glanced longingly at the TARDIS controls, the warm glow of antique brass and colourful indicator lights signalling adventure and freedom. Images flashed through his memory, his electronic brain almost fast enough to match the natural mind it housed: in spite of himself, he reviewed their history of mostly ill-fated meetings, but also potential futures… at least one of which, it was true, scarcely bore thinking about. The Master stood completely still and silent for a moment, eyes half closed, conferring with the ship's consciousness. Then he made a decision.
“Doctor, I need you to stand up straight. Can you do that?”
But the Doctor looked about to collapse if they let go of the console. Gallifreyan anatomy had many useful developments over other bipedal life-forms, but evolving a respiratory system that was spread out over the whole body rather than in centralised lungs certainly had as many disadvantages as advantages. Speaking of those advantages, though…
“Very well. Can you activate your respiratory bypass? And if it runs out, try to breathe slowly . I shall be back presently.”
The Doctor must have taken his suggestion, because the wheezing stopped before the Master got past the doorway. The bypass wouldn't last as long with inflamed breathing passages, but it might be enough to give them a bit of a rest.
The kitchen had moved itself very close to the console room, and when the Master walked in and saw an electric kettle already boiling water, he smiled and patted the wall conspiratorially.
He readied the teapot and put a tea-cosy over it, grabbed a cup in passing, and hurried back to the console room, slowing his steps only as he entered the Doctor's field of vision. He found them still standing, making an effort to breathe in, slowly, through their nose and out, just as slowly, through pursed lips. He thought with an inner smirk that when it really mattered, they’d obeyed.
The Master set down the empty cup and aimed the teapot’s spout under the Doctor's nose. “It may help to inhale the steam while the tea is steeping.”
They did as instructed, even as they avoided eye contact and shot him sideways glances. They’d designed the Master's mechanical face to be capable of great expressiveness, but that didn't mean it couldn't become as unreadable as he wanted.
When the Master withdrew the teapot, the Doctor pulled out a disreputable-looking handkerchief from some pocket and coughed deeply and painfully into it. Their breathing was only a little less laboured, but when the Master indicated the chairs at the far side of the room, the Doctor seemed only too glad to make their way over there. They gingerly stretched out their long limbs as if the upholstered leather surfaces were anything but soft and inviting: the ache in their respiratory passages must have reached most of the length of their arms and legs. The Doctor lay back, closed their eyes, and again made a visible effort to control panicked gulps of air into something resembling steady breaths.
The Master sat in the other chair and poured the tea. “Here,” his voice was almost gentle as he pushed the cup across the small table.
The Doctor opened their eyes a bit, reached for the teacup, and raised it shakily to blue-tinged lips. “It's… poisoned… isn't it?” They were still wheezing badly, but they drank some before he could think of a clever retort.
All this time alone isn't good for sharpening my wit , the Master thought. I must be losing my touch. But he had to say something.
Whilst he considered several options, the room fell silent save for the omnipresent background hum of the TARDIS and the Doctor's somewhat ragged breathing. As they continued to sip at the tea without keeling over or foaming at the mouth, some of the suspense dissipated, until it could almost have been said that the two were sitting in companionable silence. The Doctor's respiration gradually calmed, except for occasional deep coughs that drew up their whole body in obvious pain.
During one particularly bad bout of coughing, the Master reached across and laid one gloved hand on the Doctor's shoulder; their frame seemed slight and fragile under all the layers of clothing. “I need you alive,” he said when their breathing had quieted again.
The Doctor's eyes opened wide; they stared at him as if he'd confessed something important.
If he had, he wasn't going to let on. The Master withdrew his hand and slid aside an elegant cuff to indicate one of the ports where his silicone skin could be peeled off to access the electronics below: “In case of maintenance issues, of course.”
The Doctor drank up the last of their tea, the cup almost hiding the ghost of a smile that twitched at one corner of their mouth. “Of course.”