I love Martha/Master pairings this can be read on its own or as a sequel to I hate everything about you. I wrote this in about 5 minutes so it's not brilliant. This is set after the fourth series.

The doctor had just sent Donna home. It was so unfair she couldn't remember any of the wonderful things she had seen.

He had been so deep in thought he had unconsciously ended up in Martha's neighbourhood. Martha had been a good assistant one of the best. Since he was here he decided to pay her a surprise visit.

He walked up the stairs and knocked on the door of her flat.
"Martha?" he called.
There was no answer. Not bothered he used his sonic screwdriver on the lock and just walked in. He assumed she was at UNIT and decided to wait for her. He heard Martha's laughter coming from one of the rooms so he just walked in.

It was Martha's bedroom. She lay there completely naked underneath the Master.
What?
Martha and The master it wasn't possible.
"You, you're alive?" The doctor blurted out glaring at the master.
The two of them turned surprised. Martha looked scared and attempted to cover herself up. The master just looked vaguely shocked vaguely amused and very annoyed he'd obviously been enjoying himself.

"Didn't your mother ever teach you to knock before entering a room?" The master demanded cheekily sliding off Martha who was clutching the bed covers around herself.
"Why aren't you dead?" The doctor demanded.
"Not telling you big secret." The master smirked in that annoying manor he had adopted in his latest regeneration.
The doctor turned to Martha angrily, "Why didn't you tell me he survived?"
"I don't know..." Martha trailed off turning away.

The Doctor stormed out of the place. How could Martha betray him like that? He couldn't understand they hated each other he'd almost killed her and she'd been having sex with him, she'd been laughing? He couldn't look at her. He supposed he was jealous he didn't love Martha but he'd always assumed she loved him. It was insulting to think the Master had taken his place.
A few minutes before he reached the tardis he heard Martha's voice
"Doctor." Her hair was loose which was unusual and a bit messy. She wore a t-shirt and jeans. They were a bit crumpled and she wore odd shoes. It was obvious she had just dressed in a hurry.
"I know it looks bad but..."
"Looks bad? Of course it looks bad." He snapped he knew he wasn't being fair he wasn't giving her a chance to explain.
"You must think I'm a horrible slut." she whispered tearfully.
The Doctor instantly felt guilty.
He was going to say he was sorry for reacting so badly but then he saw the master. He hadn't come too close he was standing on the corner watching the Doctor and Martha. His facial expression wasn't exactly angry. But his look told the Doctor 'she is mine'.
The Doctor lost his temper and yelled
"You know what you're welcome to each other. If you want to end up dead Martha you should know how dangerous he is."
"I knew you wouldn't understand. I don't like him Doctor. In fact I hate him but I love him as well."
"You love him?"
"Yes. Im sorry."
Martha took one last look at the Doctor and ran to the Master.
He took her in his arms and kissed her.
The Master gave The Doctor a look which said
I win