Nathan isn't exactly welcome around town but he still has to do his job, which, at the moment, is tracking down the source of multiple spontaneous combustions all over town. If he and Dwight don't find the culprit soon, Haven Founder's Day promises to be very grim.
Dwight is annoyed with Nathan because he's spending too much time looking for Audrey and Jordan is annoyed with Nathan for putting himself in harm's way during his police investigations but Nathan forges ahead anyway with Duke and Jennifer at his side. With their help, Nathan is able to track the deaths to a fireman named Don Keaton. Keaton recently survived a fire when his partner did not and his ensuing guilt is activating his "trouble": recreating that fire for anyone who happens to be standing nearby.
Unfortunately, Keaton is about to be publicly honored for his bravery and the guilt that comes from accepting that award is likely to cause him to incinerate much of the town. Thankfully, Nathan finds Keaton and tries to talk him down while Dwight clears the area and Jordan tries to intervene by using her powers against Keaton. His bravery almost costs Nathan his life. As Keaton suddenly feels more guilt, Nathan's skin begins to burn. Since Nathan feels no pain, however, he is able to keep his composure long enough help Keaton understand his role in Haven's recent deaths.
Even thought the day is saved, Jordan is furious at Nathan for almost getting himself killed before Audrey can kill him, thereby ending the troubles for everyone.
Speaking of Audrey, her memories of her life as Audrey Parker are completely gone and all she knows is that she is a bartender named Lexie. Of course, her new friend William is trying to help her remember but she isn't exactly cooperating. At least, not until she realizes that she knows how to reassemble (and use) a handgun.
Back in Haven, Duke has his own set of worries. His brother Wade has been coming to Haven to help settle Duke's estate but Vince and the rest of The Guard are worried that, if he stays too long, Wade will take on the secret family hobby and become a "trouble" hunter. Duke tries to get Wade to move back to New York but when Wade catches his wife cheating, he chooses to extend his time in Haven. Indefinitely.
Colin Ferguson gives his final thoughts on the season finale of Haven.
In Haven's fourth season finale, William's plan to turn Audrey back into her original self, collides with Audrey, Duke and Nathan's plan to trap William back in the dark place he came from.
Get a look at the references to Stephen King's works from the season 4 finale.