What if instead of Irene calling Moriarty at the start of Belgravia it was Mycroft? A protective Mycroft missing one vital piece of information. A study in the gap between the Pool and the dynamic duo's next case.
I have no beta so please do ignore typos and such, but if you tell me I will fix.
Inspired by a mix of many stories and I've several ideas where to go from here, so reviews and advice appreciated.

..."You'll be hearing from me Sherlock." Moriarty's parting comment lanced. Then he turned dismissively towards away, all focus going to his phone. "So if you have what you say you have, I will make you rich.
If you don't, I will make you into shoes." Voice dropping ominously, Moriarty casually snapped his fingers while exiting.

Immediately the sniper sights disappeared from both Sherlock and John. Slight scuffling noises could be heard but in a matter of seconds silence once again reigned. John blinked twice, then managed to mumble a "Who-what?" That somehow became one syllable instead of two.

Sherlock, for once, didn't comment, thinking hard with his eyes narrowed. "Someone bargained...someone connected...Mycroft!" Shouting the last word, sending a startled John back into the wall, Sherlock pivoted and stormed out, the door thundering against the wall.

John took a deep breath to pull himself mentally together, knowing in a minute he would have to police the two oldest children he had ever met. Straightening, he turned toward the door Sherlock had just vacated, which presumably held the exit.

Two steps and John felt a familiar sensation in his right shoulder. Like the sting of an insect, the bite of the dart was noticeable even through the jumper clinging valiantly to his skin. Two strides more and John fell to one knee, starting to breathe heavily as the sedative coursed through his body for the second time that night. As his head hit the chlorine-soaked tile he couldn't help but wonder when he would finally get a cup of tea.

***

While John tried to collect himself, Sherlock threw himself out the doors and started verbally assaulting the smiling Mycroft who awaited him.

"I did what I felt I had to Sherlock," Mycroft spoke assuredly. "Mummy would be most displeased if I left you in danger."

"What you fail to realize as usual, Mycroft," Sherlock bristled "is that it was all under control. John and I had it completely..."he trailed off as Mycroft raised a hand.

"Dr. Watson is not with his lady friend?"

"If you'd use your eyes you'd know that already!" Sherlock half turned "John is right...John?"

Completing his turn, Sherlock stared with disbelief as his legs involuntarily carried him towards the school door. He managed three ground-covering strides to reach the entrance. However, as his hand grasped the cold metal handle his world rocked sideways as the interior exploded, the fireball caressing the door in counterpoint to his involuntary cry of horror.

In the ensuing commotion, no one noticed the two black sedans exiting without turning on their headlights as they disappeared into the night.