it is a long story 一言难尽；说来话长；说来话长了

【例句】
For the illumination, it is a long story.
这个启发说起来话就长了。

It is a long story. Basically, she thought I had laughed at her while I didn't.
说来话长了。简单的说呢，她误认为我嘲笑过她，其实我没有。