Beginning of Danny Phantom: The Fentons are ghost hunters who never see ghosts and are thought of as crackpots. Danny falls through floors because he can't control his powers. The trio has to fight angry lunch ladies and box fanatics. Episodes are fun adventures with lessons about bullying or w/e.
End of Danny Phantom: Ghosts everywhere. The mayor is a ghost. There is a government ghost hunting division. Danny can defeat the entire ghostzone with his voice. The trio has to fight an evil future version of Danny that straps them to a vat of hot sauce left to explode and a planet destroying asteroid. Vlad Masters tries to murder a 12-year-old.