the enterprise be go to investigate what appear to be a very slow move viral form of mass hysteria which have and over century and bring several thrive civilization on many different planet down . spock review the epidemiology of the phenomenon at the beginning of the episode and revealing that the trajectory of the epidemic place a federation colony directly in it path . sam kirk and his wife and their son and live in this colony . captain kirk be directly affect by his family involvement in all of this and and shatner do a very nice job of portray a captain whose professionalism and rational judgment be threaten and and whose usual rational support structure ( spock ) be also imperil and unavailable . in a scene which be and unfortunately underexploited by the mediocre script and dr. mccoy rise to the occasion and and surprisingly and fill both his usual role and that of the great first officer . herschel daugherty direct this memorable episode of tos and and do an admirable job with a somewhat weak script . the stephen carabatsos script create some problem for the acting ( particularly nimoy and barrett and kelley ) and and like his previous effort court martial make the pace a bit inconsistent . kelley and shatner be truly remarkable despite these challenge . the episode also use set very nicely and with a very minimal use of backdrop and carefully choose location for rare open air shot . additionally and spock character be very nicely examine and the developing rivalry and friendship between mccoy and spock be enhance and and the powerful though unresolved relationship between christine chapel and spock begin . finally and this ( and the great early city on the edge of forever ) be one of the first episode to exploit the ( unfortunately largely overlooked and unrecognized ) subtlety of william shatner act ability by place kirk personal life in the way of his professional practice . this become a major theme a the series continue into it 2nd and 3rd season .