From Longman Dictionary of Contemporary Englishbut that’s another storybut that’s another storyMENTIONused when you have mentioned something that you are not going to talk about on this occasion → story
Examples from the Corpus
but that’s another story• I did not get home till 6:00 am on sunday after the spurs game!!! but that's another story.• You could, of course, buy one of the super Chunky machines - but that's another story!• There's room for even more, but that's another story.• It also causes lucrative publicity and a scapegoat, but that's another story.• Like Birdie Walker, I survived, but that's another story. 