You mean this story: Through Blind Men's Eyes (http://ladyblahblah.livejournal.com/33581.html) by ladyblahblah. Jim's worst fear is Spock's death, Spock's worst fear is Pon Farr. And since Spock is a noble idiot who'd let himself die in Pon Farr, the aliens' gift comes in handy and Jim takes on that burden. I won't say how as to not spoil it for you. :)