We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember "dennis@home"
saying something like:

Let's see Dennis's hysterical response.

Look out the grammer police will get you.

It's perfectly correct, you ignoramus.

I thought it should read Dennis'

Wrong.

So you are lumping all the people named Dennis in one with me, what a
surprise.

What are you burbling about now?
The 'hysterical response' belongs to 'Dennis' and the apostrophe 's'
denotes such. Simple rule. If I had chosen to include all the
'Dennises', it would be, "Let's see the Dennises hysterical response."

Now get back to your signwriting business; your apostrophe' need's you.