In the fanfic, perhaps the initial situation of Harry being given too much Veritaserum is an accident. But once it starts to take effect, his every word becomes a revelation. The story might develop as his closest friends scramble to figure out who did this to him and how to stop the flow of truth. Meanwhile, the antagonists in the story see this as an opportunity. They start to manipulate the situation, using the information Harry reveals to their advantage. This could lead to a series of events where Harry is in the center of a power struggle between good and evil, all because of the Veritaserum.
It could start with someone having a malicious intent towards Harry. They might slip him the Veritaserum in a drink or some other way. Then, as he starts to spill the truth, it creates a chain reaction. People around him start to react to the things he reveals. Maybe his friends try to protect him from further exposure, while his enemies try to get more information out of him.