No one likes tourists, especially when those tourists are demons from the underworld with a penchant for torturing and killing all humans. When people say, "All hell broke loose", they probably weren't talking about They Might Be Demons. Although Hell doesn't exactly "break loose" in this book, it does take a little vacation.
Destination? Earth. And while that may sound like a bummer as it is, just wait until you discover they've chosen YOUR town as the primary get-together spot. Oh crap, SPOILER ALERT! Sorry for ruining the big surprise. But yeah, you're pretty much boned. Have fun!