▁he ▁makes ▁good ▁on ▁( with ▁A iden ' s ▁help ▁in ▁a ▁sad ▁ir ony ) ▁in ▁episode ▁ 2 . 2 3 ▁" H ero es ." ▁ ▁A ▁serial ▁k iller , ▁Clay ▁Dob son , ▁whom ▁Mac ▁helped ▁put ▁away ▁five ▁years ▁previously , ▁comes ▁back ▁to ▁ha unt ▁him ▁after ▁the ▁man ▁is ▁released , ▁thanks ▁to ▁Mac ' s ▁arrest ▁of ▁the ▁detect ive ▁( De an ▁Tr uby ) ▁who ▁took ▁his ▁conf ession . ▁Mac ' s ▁single - mind ed ▁intensity ▁in ▁his ▁renew ed ▁purs uit ▁of ▁this ▁k iller ▁makes ▁him ▁short - temper ed ▁with ▁his ▁concerned ▁cow ork ers . ▁After ▁the ▁CS Is ▁discover ▁one ▁of ▁Dob son ' s ▁vict ims ▁is ▁still ▁alive , ▁Mac ▁charges ▁after ▁him , ▁alone , ▁corner ing ▁him ▁on ▁a ▁roof ▁of ▁a ▁tall ▁building . ▁Dob son ▁comes ▁crash ing ▁to ▁earth ▁moments ▁later , ▁fat ally ▁impact ing ▁on ▁the ▁h ood ▁of ▁a ▁police ▁cru iser , ▁only ▁feet ▁away ▁from ▁F lack ▁and ▁some ▁uniform ed ▁officers ▁arriv ing ▁as ▁backup . ▁His ▁hands ▁are ▁cu ff ed , ▁and ▁Mac ▁gaz es ▁down ▁from ▁the ▁roof ▁in ▁horror ▁( ep is ode ▁ 3 . 2 1 ▁" P ast ▁Imper fect "). ▁Flash back s ▁in ▁the ▁following ▁episode ▁reve al ▁that ▁Dob son ▁intention ally ▁fell ▁from ▁the ▁roof , ▁telling ▁Mac ▁that ▁if ▁he ▁went ▁down , ▁he ▁would ▁take ▁the ▁detect ive