whole other story

Whole other story is an idiom which is usually said to say what you are speaking of is something else entirely. Typically it is said during a story or conversation in which during it, you or someone else speaks about a certain subject or situation that relates to this topic but is swaying from the point of your story. By saying that it is a whole other story, it is to say you will not speak of it now because it will start an entirely new story or topic of conversation.