Plot summary
Several weeks after the death of Abbie Mills, Ichabod is held and questioned by a man about his past and is shown detailed pictures of his investigations and of Abbie. Ichabod distracts and knocks out the man, and discovers a reference to the Lincoln Memorial before escaping. Meanwhile, Homeland Security agent Diana Thomas and her partner Eric investigate the headless statue of Lincoln at the memorial, and Eric is attacked and killed by a demon, which nearly kills Diana as well before Ichabod chases it off. The two form a partnership and travel to "Agency 355", a division of the government that deals with paranormal threats.
While there, Ichabod and Diana meet technician Alex Norwood and historian Jake Wells, and learn that the demon is from 1865, and was summoned by John Wilkes Booth, who used it to kill Lincoln. They learn that the demon is drawn to the American flag, and is weak against copper, which they use to set a trap. At the last second, Jenny shows up and destroys the demon, saving Jake's life.
Credits
Adapted from Wikipedia contributors, Sleepy Hollow season 4. HTML formatting and reference markers removed.