It's a long story.

Meaning

The event is complex and would take a while to explain.

Examples

• John: How did your meeting go?
• Alice: <b>It's a long story</b>. Let's just say it was eventful.
• John: Oh? Tell me everything!
• Alice: Well, it started with a power outage...<b>It's a long story</b>. Maybe another time.