In the run-up to Zelda's wedding, Dan becomes obsessed with a dying man's final words, and Jasper finally decides to act on his own feelings.

After Season 3’s shocking finale, Dan and Jasper are consumed by grief. Dan throws himself into work while the Professor hides in his lectures, avoiding the police at all costs. But Helena, Jasper’s therapist, helps him face back up to his police work and the team tackles complex and dangerous cases. But through the danger and mystery, love is very much in the air.