Jimmy Carr hosts this Horizon science film, pulling in researchers to pick apart what laughter actually is, biologically and socially. Inside UCL's anechoic chamber and an MRI scanner, scientists compare the brain and body during laughing versus speaking, showing how laughter hijacks the ribcage and breath in ways that can genuinely drop you to the floor. Robin Dunbar tests whether laughing raises pain thresholds through endorphin release, and traces a line from ape grooming to humans using group laughter as a kind of remote tickle. Sophie Scott's work shows how the brain tells real laughter from a performed one, and why a single contagious laugher can set off an entire room. Peter McGraw explains his Benign Violation theory, his account of why something edgy reads as funny while a genuine tragedy never does. A detour into rats that chirp when tickled, and the antidepressant research that followed, rounds out the case that laughter is less a reaction than a piece of social machinery.