It's a long story.

Meaning

The explanation is lengthy and complicated.

Examples

• John: 'Why are you so late?'<br>
• Alice: '<b>It's a long story</b>, but let's just say I had a series of unfortunate events.'
• John: '<b>It's a long story</b> involving a last-minute project and a fortunate coincidence.'
• Alice: 'How did you manage to get that promotion so quickly?'<br>