It's Halloween time at The Jeffersonian, as the team gets a surprise visit from newly minted FBI Agent Abbie Mills and Ichabod Crane, who have traveled from Sleepy Hollow to claim a headless body already under investigation by Brennan and her team. The new foursome must figure out how to work together to discover who the 200-year-old headless corpse is, and how it is linked to a contemporary murder victim. Meanwhile, Booth and Brennan come up with the ultimate Halloween pranks to play on each other.