Spoilers for the end of "The Six Thatchers". Everything that follows the first line is pure wish fulfillment on my part for S4. This will be a two-parter with mentions of infidelity but no MCD.

"Sherlock," Mary said warningly.

He paused in mid-deduction, instantly understanding what she was trying to tell him: if he continued in his scathing commentary, Vivian Norbury would snap and do something they'd all live - or not, if that was actually a gun she had in her hand-bag - to regret.

So he stopped. He and Vivian eyed one another warily until John, Mycroft and Lestrade showed up, along with a pair of officers who very efficiently cuffed the secret-selling secretary (as John would call the case in his blog, naming no names in the interests of national security) and led her off to jail.

Sherlock accompanied the Watsons back to their flat, took several photos of his sleeping god-daughter, fired every single one of them off to his brother under encryption, swept Molly up into the cab with him back to London, and left the little family he'd managed to protect safe at home.

Or so he thought.

Can I bring Rosie by for the rest of the day? JW

Of course. Although I'm surprised the clinic called you both in for work, knowing your personal situation. SH

Normally I'd love telling you you're wrong. Will explain later. JW

Sherlock raised a brow at John's cryptic answer. His first instinct – to fire off a new series of deductions, most of them revolving around John's sister Harry and her unfortunate propensity for alcohol – was swiftly quashed. If it was Harry, John would have said so outright. John wasn't cryptic. Ever.

So with a combined sense of anticipation and trepidation, he waited for John and Rosie's arrival…and quite heroically kept himself from firing off a text to Mary demanding to know what was going on.

oOo

"John?"

Mary frowned as she entered the flat. It was immediately obvious that John wasn't there – which meant Rosie wasn't there, either. A run to the shops? A case? She looked at her phone – nope, no missed calls or texts. She sent her missing husband a quick query, only to freeze as she heard the quiet buzz of his phone nearby. A quick glance revealed that it was sitting on the coffee table, with a note propped up next to it.

Nerves prickling, she moved swiftly forward. Their life hadn't even settled back down for a full day, for Christ's sake!

She didn't relax upon seeing the familiar handwriting on the folded sheet of paper; just because John had written the note himself, it didn't necessarily follow that he hadn't done so under duress.

However, as she read what he'd written, she almost – almost – found herself wishing he'd been kidnapped.

My dearest Mary, I've left my phone for you because, well I'm too much of a fucking coward to do this in person. "If you love me, don't read it in front of me because you won't love me when you're finished, and I don't want to see that happen." Sound familiar?

She stopped reading, taking a few long, slow breaths, bringing her racing heart back under control before taking up the letter again.

Please read the series of texts you'll see under the contact name 'E' and then, when you've finished, read the second note I've left under the phone. Please.

She looked down, a mere flick of the eyes to confirm the presence of another folded piece of paper tucked beneath John's mobile, then finished reading the first note.

I'll be at Baker Street. Waiting. For however long it takes. Sherlock and Molly have Rosie; you can pick her up any time you like at Molly's flat. Even if you don't believe me –and I won't blame you if you don't – just remember that I love you. No matter what I've done, I love you. John

It took her a full minute to actually pick up John's mobile, to do as he'd said and scroll down through the line of texts. She read them all, from start to finish, then dropped the phone onto the coffee table.

This isn't a good idea.
I'm not free.
Things won't end well.
It was nice to get to know
you a little. Sorry.

That was the last text.

She picked up the second note with shaking hands. Unfolded it. Read it. Dropped it on the table. Picked up John's mobile, with its lockscreen picture of the two of them holding Rosie. Considered hurling with all her strength against the wall and watching it smash.

Instead, placed it carefully into her handbag, exited the flat, locked the door behind her, and headed for Baker Street, the words from her husband's second note still whirling through her mind.

It was never physical, I swear. That's no excuse; it was still wrong, I know it was wrong, but I just…fuck. There's no excuse. She was sweet and pretty and I enjoyed the attention and knowing I was still attractive even if I was a middle-aged husband and a dad. I let my ego override my common sense and this was the result. Just know that it was never about not loving you. It was only about me being selfish and stupid and now it's about me realizing how selfish and stupid it was. I may have lost you forever and if I did it's my own fucking fault and I can say I'm sorry a million times over but I know I won't deserve it if and when you decide to forgive me. Anything else I guess I should say to your face so I will. I love you Mary, and I will do anything to make this right. And if there isn't anything I can do, that's your decision and I'll respect it. But I'll never stop loving you. John.

Right up to the moment she entered Sherlock's flat, Mary had no idea what she was going to do. But as soon as she saw John's stricken face, her racing mind stilled, her focus cleared, and with a few quick steps she was standing directly in front of him.

He never saw the punch coming; She laid him out flat. Blood streaming from his nose and split lip, he stared up at her from his prone position on the floor. "Yes," Mary said calmly as she stared right back down at him. "You deserved that. And we'll talk about it later. After we go pick Rosie up." Then she extended her hand, waited for her errant spouse to reach up and take it. Helped him to his feet.

Let him hold her hand as they headed out. Because if there was one thing Mary Watson had learned, it was that no one was perfect – and that no matter what mistakes she and John made, now or in the future, she would rather they dealt with those mistakes together than apart.

Always.