Tomorrow, a bar inspired by the grand tradition of unsupervised house parties will open in River North.

It will be in the basement of Henry’s. It will be called Rec Room. And it will be all kinds of insane.

Here’s what you’ll do:

You’ll make reservations if you have more than five people who want private seating, punch bowls and a bar cart stocked to your specifications.

You’ll just show up if you don’t.

You’ll enter Henry’s and head downstairs to a room marked by a giant skull.

You’ll brace yourself.

You’ll walk in and survey the land.

You’ll look at the Christmas lights, the disco ball, the mismatched retro furniture and all the Milwaukee’s Best cans and think of Grandpa’s rumpus room circa 1978.

You’ll finally fully comprehend Grandpa’s rumpus-ing prowess.

You’ll order bottles of housemade margaritas or gin and tonics from the cash-only, walk-up window.

You’ll be so thankful for hitting the ATM before you came.

You’ll dance to a jukebox stuffed with only 45s and playing everything from James Brown to LCD Soundsystem.

You’ll raid that refrigerator in the corner at midnight.

You’ll snack on things like fried chicken, popsicles, cold pizza and/or barbecue that you find inside it.

You’ll be very glad Henry’s parents are leaving him home alone tomorrow night.