it's a long story

The story is very long and convoluted (and perhaps too long to tell the whole thing). A: "So how did you and Jim meet?" B: "It's a long story, but the short version is that we met in college." A: "Why were you running across the courtyard in your underwear yesterday?" B: "Ugh, it's a long story. I'll tell you tomorrow."

it’s a ˌlong ˈstory

(informal) used to say that the reasons for something are complicated and you would prefer not to give all the details: ‘So why did you leave?’ ‘Oh, it’s a long story — I’ll tell you some other time.’