You dose Simon with DIME-53.

You wait until late at night, when everyone is sleeping, then young down to the basement and look for the cooler.
It's just where you found it last time, with the same contents. Carefully, you get two thermos and a hip flask from the kitchen, and then you get to work. The liquid labeled POLE-21 gets emptied out into one thermos and most of DIME-53 goes into the other. The final portion of DIME-53 goes into the hip flask. You don't know yet what you'll do with the rest, but this quart is for ****.

You refill the vials with water and put them back. Hopefully, they'll just figure the **** were duds. You take the thermos out into the dead of night and you make sure you aren't followed as you bury both thermos at the foot of a random tree in the nearby park.

The hip flask gets hidden somewhere much closer. Your wooden backyard fence easily opens aside, to the backyard of Simon's house. Both of your families regularly come and go through this "door", and you both know where the spare house keys are hidden, like good neighbors and close friends should. In case of emergency. The hip flask will stay next to Simon's spare key until morning.

You sleep nervously, but get up before Heather wakes. Slipping out and into Simon's backyard, you retrieve the flask and the key. You manage to sneak in, overhearing Simon and Penelope talking. They're morning people, early risers.

But Simon still needs a hot cup of coffee in the morning anyhow. He's already got one going, reading the paper in the kitchen.
It isn't long before Penelope interrupts and lustfully tells Simon that a quickie to start the day would be nice. He quickly leaves the still hot coffee behind as he undresses and follows his wife into the bedroom. Perfect.

Once you hear the tell-tale sounds of fucking, you creep up to where Simon was sitting.
You pour the contents of the flask into the coffee - he's not gonna let it go to waste. One quick fuck and he'll back to recharge for Round 2.

You sneak out, hide the key again, chuck the flask into your own backyard and quietly go over to the bedroom window to check on the happily married couple.
As expected, they're still at it.
No matter how many times you witness it, you still feel awed and belittled by Simon and Penelope fucking.
Wumpa, wumpa, wumpa, Penelope gets power-fucked on that brawny olive meat stick, screaming out that he's a stud. The hot Korean babe's enormous boobs bounce up and down, in time with the whopper's thrusts, drenched with sweat. Her breaths are coming in short bursts. Her head is resting on one of her husband's broad shoulders with her eyes screwed shut holding back tears from the sheer erotic pleasure.
It comes about suddenly and almost without warning. After one significantly forcefully thrust, an explosion occurs deep down inside Penelope. The sensation travels up her spine to the top of her neck. Her entire body becomes rigid all at once. This is the moment she has waited for.
Penelope's pussy contracts violently around Simon's cock. The already tight hole becomes like a vice as it grips his steel shaft. The dam breaks, and Simon's chunky balls contract.
It's not long before they both cum, exploding a puddle of juices out.

With Penelope satisfied on the bed, Simon walks back still naked.
Taking care to not make a sound, you sidle over to a window letting you peek into the kitchen.
There, you can see Simon grabbing his coffee again, still naked, letting it all hang out...
He sits down on the couch, legs splayed and giant genitals displayed...
He chugs the coffee, swallowing the rest of the now lukewarm drink in a few gulps...

Simon puts the mug down...and seems to quickly fall asleep?
You look around, making sure you yourself aren't being observed. When you turn back, you instantly notice Simon's flaccid penis no longer extends halfway to his knee. Instead, it is about 3 quarters as long as that. The muscles and veins in his length are less strong and pulsing as well.

Just as you process this, Simon's dong shrivels a bit more - now it's half as big as you usually see it. His nuts are smaller, too. It was a burst of transformation, but it definitely happened.

Another burst of shrinkage takes ten seconds to happen - this one is even more dramatic. 4 inches long, you guess? Thinner than half his previous girth, too. Your neighbor is now the stereotypically underendowed Asian, bigger than yours but much smaller than what he had a minute ago. His balls are pretty small now, too.

The last burst reduces him to your size. Thin as a straw, short enough to be wanked with two fingers, and already leaking drips of cum. You could cup it all completely in your hand, no problem. He still sleeps blissfully on, completely unaware.

Mission accomplished.

On the way back, you pick up the flask and take it inside to be thoroughly rinsed.