“What do you mean, the Hellmouth is missing?”
“Checked the basement,” said Buffy. “Nothing there. No sigil, no creepiness, nada.”
“Extraordinary,” Giles said, several minutes later. “Also completely impossible. It’s a complete mystery to me.”
A silver portal irised from nowhere, spat two teenagers into the library, and disappeared. The girl waved a badge. “We’re here to get your Hellmouth back.”
Buffy blinked. “And we’d want it back why?”
“Hey, wait,” Xander said suddenly. “You’re Ivy and Zack, right? From the Acme Detective Agency?”
“That’s us,” Zack confirmed.
“But that means—”
Ivy nodded. “Carmen Sandiego has stolen the Hellmouth.”
The Buffyverse belongs to some combination of Joss Whedon, Mutant Enemy, and the Kuzuis; Carmen Sandiego belongs to Broderbund Software, DIC, and their corporate successors.