Nightmare
The Master had noticed one thing in the time he had traveled with the Doctor.
She didn't sleep.
A human wouldn't notice it, especially one as oblivious and easily distracted as Donald Noble, but to the Master it was actually quite obvious. When he was supposedly sleeping in his room, he was actually just waiting for the Doctor's consciousness to slightly faded away to show that she was asleep. Not because he was spying on her and plotting to kill her as she slept, but because traveling on the TARDIS had made the Master realize how much he missed the company of his own species and the Doctor was one of the few he could stand, never mind that she was the only other one still alive other than him.
The Master had finally had enough after a month of the Doctor's insomnia. Time Lords may not need as much sleep as humans but he knew that she was tired; she hadn't slept once whilst he had been here, and probably hadn't slept for a while before he had arrived on the TARDIS.
Walking through the complex TARDIS corridors to the console room was quicker than normal due to the TARDIS worry for her Doctor. When he arrived in the console room the Doctor was pointing her screwdriver at the console which was furiously throwing sparks at her. The Master rolled his eyes when he realized that the TARDIS was trying to stop her from messing with the controls and pushing her to sleep. Of course the Doctor was being stubborn and refusing.
The Master continued to stand there, only moving towards the Doctor when she had thrown herself onto the small sofa nearby. Her face was filled with annoyance at the TARDIS which only increased when the Master sat next to her.
"If you listen to her then you wouldn't have to go through this," He says quietly but knows that she had heard him when she rolled her eyes. "I mean it. You haven't slept in a long time, even I've noticed," He says but the Doctor remains silent. Stubborn as usual.
"What's wrong Theta?" She looks up at him sharply when he says that and the Master raises an eyebrow at her, staring straight back at her until she sighs. Her whole body deflated and she leaned against the Master, leaning her head against his shoulder. This surprised the Master; she seemed so different from the Doctor he was used to seeing and knew that something was seriously wrong for her to be acting so out of character.
"I can't sleep Koschei," The Master didn't comment about the use of his name, instead choosing to hesitantly wrap his arm around her shoulders as she closed her eyes tightly.
"What's wrong?" He asks again and was astonished at the change in him. To humans and aliens threatening them, he was the same old Master. He snapped and threatened and only listened to the Doctor's orders. Grudgingly. However, around the Doctor - when they were alone - he was Koschei again. Theta's best friend.
"The nightmares," She whispers and her eyes were clenched shut as she wrapped his arm around his waist in return. The Master unconsciously tightened his arm. He could emphasize with her; he suffered from the same nightmares of the Time War but knew that they would be worse for the Doctor
They were silent for an unknown amount of time. They sat their with his arm around her shoulders and hers around his waist. It took the Master fifteen minutes to realize that the Doctor had fallen asleep.
And for the first time in a long time, the Doctor had someone to talk to about the nightmare when she woke up.