Lexie, the new girl in town, joins Nathan on a hunt for a deadly trouble that changes its victims' personalities.

When Audrey and dozens of Haven citizens are affected by a mysterious illness, the symptoms threaten to expose Haven’s secrets to a dangerous newcomer.

Haven’s heroes are torn over how to deal with an inquisitive CDC scientist, who could expose the secret of the troubles to the world, but may be hiding her own dark agenda.

Desperate conditions in Haven lead to a terrified population that's difficult for Dwight to maintain control over; at the same time, Nathan assembles a task force to restore power to the town; and Audrey tries to put an end to the trouble once and for all.

Dwight and Charlotte work to clear Nathan's name when the citizens of Haven put the lawman on trial. Hoping to buy them time, the brothers Teagues moderate the courtroom proceedings, while Audrey continues her quest to solve the darkness troubles.