It varies. Sometimes a medium claims to be based on a true story but takes liberties for dramatic effect. Other times, it's purely fictional with no connection to reality.
I'm not entirely sure which specific 'the medium true story' you're referring to. There could be many works with such a title. It might be about a medium in a real - life context, perhaps someone who claims to communicate with the spirit world and the real events and experiences associated with that person.
The show Medium was not based on a true story. It was created by the writers' imagination and creative concepts. The characters and events were fictional constructs to entertain the audience.