Elementary my dear psychopath!

HOLMES AND WATSON STAND WITH INSPECTOR LESTRADE FACING THE VILLAINOUS MORIARTY WHO IS SAT IN A CHAIR IN HANDCUFFS

LESTRADE
So the crime prince of crime turns out to having descended to naught but a vicious serial murderer. Truly Holmes how the mighty have fallen.

WATSON
Yes but Holmes how could you know this brilliant monster was hiding his poor victim under the floor board? Without removing so much as the smallest plank?

HOLMES
It was elementary my dear Watson.

MRS HUDSON WALKS IN THE DOOR WITH A WHEEL BARROW WITH SEVERAL CORPSES IN IT

MRS HUDSON
It's because he's got the bleeding flat above us!

Hey I know this is a bump but was this really that bad?

Maybe make it plain the ceiling has come down by having Mrs Hudson covered in plaster.

You could do with re-reading Lestrade's first sentence.

It's a good joke but I was too distracted by the whole Sherlock theme. Would it work better with ordinary police?

Both good points I was originally thinking with regular police and may rewrite as such