Disclaimer- plot is mine, characters ('cept for ones not in book) aren't

Note- okay, so this is yet another Minerva/Dumbledore story, with some Tom Riddle thrown into the mix. It's about Minerva's first years as a professor

Chapter one- A Vacant Position
"Welcome Minerva, tabby dear, welcome to Hogwarts!" Albus said warmly as he gave the new arrival a hug. "It is so good to see you again." Minerva smiled at him. "Though I do declare you are no longer my little Tabby anymore." Minerva laughed.
"I can hardly believe it myself." She admitted. "I do hope I'm qualified, and am a good teacher."
"Of course you will be, I hired you didn't I?" Albus told her sternly, but laughed. He smiled broadly at her. "And I know you will be an excellent head of Gryffindor house."
"I do hope so." They began to walk to Gryffindor tower. "This is so wonderful Professor Dumbledore. I haven't been here since my seventh year." Albus laughed.
"Please, call me Albus. If it helps, I am just as nervous as you are. This is my first year as headmaster. I really don't want anything to go wrong, the only problem at the moment is a vacant position. We still don't have a potions professor, and head of Slytherian house. However, Headmaster Dippit said he had found one and was sending him here today." Albus frowned as he glanced out the window and saw the setting sun. "I wish he would get here soon though. Oh, here we are." Albus said happily. He introduced her to the portrait as Professor McGonagall, Head of Gryffindor House, and the portrait gladly let them into the tower. They walked into the empty common room.
"Well, here it is!" Albus announced. Minerva gasped in surprise.
"It hasn't changed a bit in the three years I've been gone!" Minerva exclaimed as she walked around the room. Albus watched her and smiled. He was so happy she had accepted his offer to come to Hogwarts and teach Transfiguration. He had missed her in the three years she had been gone. His most fond memories of being the transfiguration professor were of training Minerva to be an Animagi. They had started in her fifth year, right after O.W.L.s. At the end of her seventh year, they had succeeded, and Minerva could turn into a tabby cat at will. In that, he had begun to call her Tabby. He turned to face her, and saw, with a smile, that she had transformed.
"Tabby, I must say you look far more docile as a cat." He said. The cat meowed. "I didn't say harmless!" he quickly added, amending his previous statement. "I am fully aware that you have claws. I am simply saying that you look more docile, that is all." He went over, picked up the cat, sat on the couch and cradled it. "However, my love, you don't look as beautiful as you do as a human." The cat was still for a moment, and Albus blinked. When he opened his eyes, Minerva was in his arms. Slowly, he kissed her and she kissed him back. They stared at each other for a long moment. Albus put Minerva down gently on the couch, and sat beside her. They stared at each other for another minute, then Aberforth came stumbling into the room. Aberforth, at 13, was going to be a third year when the term started, and had come early with his brother. He had been excited to come three weeks early to school, but had been bored so far. He ran up to his brother and Professor McGonagall.
"Albus, Professor, there is a man in the great hall, he says he is to be the potions professor, and wants to see you." Albus stood up, and Minerva did too.
"Thank you Aberforth." Minerva said softly. Albus helped her through the portrait hole, and then followed her out. Aberforth watched them leave. They walked closely together, oddly. Weird, thought Aberforth. Nah, he thought, it couldn't be. He wouldn't, and besides, she would never... Hmmm, no, just not possible, thought the boy. Aberforth returned to his rooms and pushed his brother and the Professor McGonagall out of his mind.
Albus and Minerva walked down the corridors to the great hall.
"I wonder who it could be. Dippit didn't say who he was sending." Albus mused. He looked at Minerva.
"Perhaps he is from Durmstrang or Beaubaxtons?" she proposed. She did not meet his eye. They reached the great hall and entered.
"Minerva, what happened before-"Albus began, but he was interrupted by a voice at the opposite end of the room.
"Hello back there! Professor Dumbledore, I mean, Headmaster Dumbledore, is that you?" the voice came closer, and a tall thin man appeared. He had dark brown hair and a handsome face. His eyes were a brilliant green. Dumbledore and McGonagall walked closer toward him.
"Headmaster Dippit sent me to be the potions professor sir. I don't know if you remember me, Professor, I am-"the man stepped into the light and Minerva gasped and ran toward him.
"Tom! Tom Riddle! Oh, Tom, I hadn't heard from you in so long I thought I would never see you again." She said as they embraced.
"Minerva McGonagall, Dippit never said you taught here! Yes, I was gone, but I came back to England for this position. What amazing circumstances bring us both back to Hogwarts, together." They kissed.