Another soldier in the army of evil, the Sandman, seeps into the dreams of the residents of Sleepy Hollow. By infiltrating their minds at night, he tortures them to the brink - all because they, at one time, turned a blind eye to justice. When the Sandman seemingly sets his sights on Abbie, she is forced to begin to face her past - and her institutionalized sister Jenny. With Ichabod by her side, and her life at stake, the duo must try to defeat the Sandman and continue their fight against evil.

Lt. Abbie Mills and Ichabod Crane search for Abbie's estranged sister, Jenny Tag Team, who has escaped from a Sleepy Hollow psychiatric hospital. In a game-changing episode, which includes flashbacks to the real Boston Tea Party, Abbie and Ichabod discover more about the evil they are facing... and finally learn its name in the haunting.

When Ichabod Crane goes missing, his wife, Katrina, appears to Lt. Abbie Mills in a vision warning her that the Headless Horseman will return to Sleepy Hollow by nightfall. A frantic Abbie seeks help from the mysterious Henry Parrish, who holds vital clues about how Crane can vanquish his dangerous blood tie to the Horseman. Meanwhile, an encounter from Crane's past weighs heavily on him, and details about his first encounter with Katrina are revealed.

Ichabod and Abbie discover a massive secret buried within the pages of George Washington’s Bible and unearth the shocking true nature of the founding father’s death. Meanwhile, Captain Irving makes a near-impossible decision for the sake of his beloved family that drastically changes his future.