) ▁to ▁learn ▁the ▁truth ▁behind ▁Sydney ' s ▁apparent ▁death . ▁During ▁Season ▁ 3 , ▁Jack ▁acts ▁as ▁a ▁far ▁more ▁support ive ▁father - figure ▁to ▁Sydney , ▁in ▁response ▁to ▁her ▁de pression ▁and ▁emot ional ▁isol ation ▁during ▁that ▁season . ▁ ▁At ▁the ▁start ▁of ▁Season ▁ 4 , ▁Sydney ▁lear ns ▁that ▁Jack ▁killed ▁Ir ina . ▁Even ▁though ▁he ▁did ▁it ▁because ▁Ir ina ▁had ▁h ired ▁a ▁hit ▁man ▁to ▁kill ▁Sydney , ▁his ▁action ▁causes ▁yet ▁another ▁r ift ▁to ▁form ▁between ▁Jack ▁and ▁his ▁daughter , ▁a ▁r ift ▁ag gra v ated ▁when ▁both ▁agents ▁are ▁rec ru ited ▁by ▁Slo ane ▁into ▁the ▁new ▁C IA ▁black ▁o ps ▁ag ency , ▁Author ized ▁Person nel ▁Only ▁( AP O ). ▁By ▁the ▁second ▁half ▁of ▁the ▁season , ▁this ▁r ift ▁has ▁begun ▁to ▁he al . ▁ ▁Jack ▁Br ist ow ▁contract s ▁radiation ▁poison ing ▁during ▁the ▁events ▁of ▁the ▁fourth - season ▁episode ▁" N ight ing ale " ▁when ▁he ▁entered ▁a ▁radio active ▁area ▁in ▁order ▁to ▁save ▁Sydney ' s ▁life ▁during ▁a ▁mission . ▁He ▁initially ▁keeps ▁his ▁condition ▁a ▁secret ▁from ▁his ▁AP O ▁colle agues ▁except ▁for ▁Marshall ▁( who ▁figures ▁it ▁out ▁and ▁confront s ▁Jack , ▁although ▁Marshall ▁later ▁revealed ▁it ▁to ▁Sydney ). ▁Br ist ow ▁is ▁seen ▁consult ing ▁with ▁a ▁doctor ▁who ▁tells ▁him ▁the ▁condition ▁is ▁terminal . ▁Not ▁accepting ▁this ▁pro gn osis