The phrase "fascinating backstory" refers to a very interesting and engaging history or background of a person, place, or thing. It suggests that the story behind it is captivating and holds great fascination or intrigue. Full definition
It was not until I stood at the desk of the shelter to adopt Ozzy that I heard his very fascinating backstory. (okcpetsmagazine.com)
The Ray - series has a morbidly fascinating backstory to it, light and understated though it may be. (hardcoregaming101.net)
But I'm also an admirer of Johnson — I've always found him to be an intelligent, shrewd, engaging and humorous politician, with superb communication skills and a fascinating backstory. (newstatesman.com)