I can't live with writing Sherlock/John cheating on each other so it's going to be Mary.
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John hummed softly, flipping the eggs and bacon. “Rosamund! Mary! Breakfast is done. Quickly, Rosie, you have to get to school.”

His daughter hopped into the kitchen, wearing a cute dress with flower designs and a brightly coloured backpack, which in the corner ‘Genius was here’ was written in black thread. She admired how Sherlock sewed that onto there, so she had taken a yellow marker and outlined it so it was more visible.

She hopped up onto the chair, “Can Daddy come to school today?” She asked, tilting her head.

“No, not today. Sorry sweetie, gotta go to work.” John said, giving Rosamund her breakfast.

“Awww, okay Dada.” She ate her breakfast and then got back out of the chair, walking up to John and hugging his legs. “See you when I get back!”

John bent over, kissing her forehead and ruffling her hair. “See ya!” He said. She giggled and ran out to wait for the bus. John sighed softly and looked around, “Mary!” He said.

There was no response. John wondered where she was, so he called her. She picked up. “Hello, John,” she said.

“Who's John?” A rough sounding voice asked on the same line of the conversation as Mary, sounding in the same room. She excused herself and walked into another room. “Is Rosamund on the bus?” She asked.

“Yes. Who was with you?” John asked.

“Nobody.” She said.

“Mary--” John was cut off by the voice.

“C'mon baby don't take a phone call in the middle of sex.” The rough sounding man said.

“Mary!” John growled.

“S-sorry, John, I--”

“Don't even talk to me!” John said, hanging up.

Cheating, his wife was cheating on him. He didn't know who was with her, and he needed to talk to someone.

Rosamund goes to school for seven hours, so John had a while. He called Sherlock.

“Hello, old friend.” Sherlock said.

“Sherlock,” John said, walking to the couch and laying down. “I have a problem.”

“Of course you do. You're Sherlock deprived. It's unhealthy for you to not contact me for a month. You could call more.”

“I'm sorry, Sherlock, Rosamund just started started school. That's not really a good excuse but--”

“Don't you have work?” Sherlock questioned.

“I'll call in sick. Listen, we need to talk, I have nobody else I trust more than you.” John sighed quietly.

“Okay, talk.”

“Mary cheated on me…” John mumbled. “Can I please come over?”

Sherlock was silent for a moment. “Of course.” He said.

“Thank you. Bye.”

“Bye.”

John hung up. He sighed and got to the door, putting on shoes and a jacket. He walked out and got a cab, telling the driver were to go.

A few minutes later, John arrived at 221B. He ran upstairs and walked to Sherlock, hugging him tightly.

“Okay, okay, calm down.” Sherlock mumbled, awkwardly patting John's back. “She cheated on you. That's okay, so many people cheat. You just need to get revenge…”

“How?” John looked up, staring at Sherlock.

“Well, I might have oooone idea..”

------ Six Hours Later

“I better be going to Rosamund now.” John said, sitting up.

“So soon?” Sherlock asked.

“It's been hours.” He stretched his arms over his head. Sherlock grinned.

“Alright,” Sherlock sat up and kissed John deeply, with the other man returning the kiss. “See you tomorrow?”

“Have to go to work. I'll see you again in the weekend,” John hopped out of bed and picked up his clothes, quickly getting dressed.

“Will you move back in when the divorce happens?” Sherlock asked, drawing little circles onto the bed sheet with his pointer finger.

“Of course. I never thought you could be so loving. This was an absolutely wonderful night. Thank you, Sherlock.”

“You're welcome. Do you feel bad at all?”

John shook his head, “Not at all. She was the first one to cheat, after all we’ve been through! I don't want to be with her. Better get back home before Rosie returns from school.”

“Alright, see you.” Sherlock winked and blew a kiss. John smiled sweetly and left, going back home.

Three weeks later, John and Rosie were living back at 221B with Sherlock.