For the first time ever, it's Halloween in the Big Brother house, and you can expect the unexpected lots of tricks and treats.

Since Monday is Veto day on Big Brother: Over the Top, the comp is Halloween-themed, featuring a ghost! Clementine O'Grady is haunting the BB house, and to appease her, the Veto players -- HOH Danielle; nominees Shelby and Whitney; America's nominee Scott; and Jason and Justin (the latter of whom was Whitney's houseguest choice) -- have to retrieve her three favorite items from the house in the dark, with only a red headlamp to light the way, in the shortest amount of time.

What are those three items? Well, grab your Ouija board, sit back, don't relax too much, and learn about The Haunting of the Big Brother House. As you can see in our exclusive sneak peek, Clementine was a wee Irish immigrant who, way back when, moved to a house on the same plot of land as the BB house. The only things that could calm her down during her fits were her rattle, her hairbrush, and her doll named Abigail. Unfortunately, an innocent trick went awry, and Clementine ended up murdering her whole family. Hopefully she won't kill the houseguests if they can't find her three favorite things.

Fall TV Popularity Contest: Vote for your favorite new shows!

And, because it's not Halloween without some trick-or-treaters, we can also exclusively reveal that four past houseguests are dropping by: Season 13's Lawon, and Season 17's Liz, Julia and Austin. Too bad the Austwins aren't back for a BB Chopped comp, but Jason will surely be happy to see them.

The Veto comp takes place at 4 p.m. ET / 1 p.m. PT. Big Brother: Over the Top is currently streaming on CBS All Access.

(Full disclosure: TVGuide.com is owned by CBS.)