It's hard to say for sure. It might incorporate elements of truth but could also be a completely made-up tale with a similar theme to real-life incidents.
Since it's based on a true story, perhaps it's about someone who is facing a difficult situation, like a terminal illness or a dangerous living condition. And they are sort of hidden in plain sight, meaning their situation isn't immediately apparent to others. Maybe they are trying to hide their pain or the people around them are just too preoccupied to notice. It could be a story of a person's struggle that goes unnoticed until a certain point.
The 'dying in plain sight' true story could potentially be about various things. It might be about a situation where someone was in a dire state, perhaps ill or in danger, and yet those around them didn't notice or take appropriate action. Maybe it's a story of neglect in a public or common setting where the signs of a person nearing death were visible but overlooked.
There's no way to determine the main characters without more information. It might be a victim of some sort and those who could have helped but didn't. Maybe it was a homeless person on the street and the passers - by were the other characters in this sad story.