I'm not sure if it's a horror movie just from that statement. Just because it's based on a true story doesn't mean it's horror. It could be a drama, a thriller, or even a documentary - style film.
Since it's based on a true story, it may draw from real - life events that are both inspiring and harrowing. For example, it could be about a person who overcame great odds to achieve something. Or it could be about a situation where the truth was hidden for a long time and then finally revealed. It might also involve the experiences of ordinary people caught up in extraordinary circumstances.