The basic plot: Apparent murderer who’s either insane or mind-controlled tries to cover up his crime and stay one step ahead of the police. Killer and cops are both controlled by the player.
Jumps the shark when: Protagonist Lucas Kane tries to rescue his ex-girlfriend from the mysterious Oracle, only to watch her die when both of them plummet from the top of a roller coaster. From there, the plot rapidly devolves into bizarre supernatural nonsense about apocalyptic conspiracies, a looming Ice Age, diabolical artificial intelligences, Mayan priests and psychic powers. It also involves more than one Dragon Ball Z-style fight, in which skinny IT manager Lucas flies around New York City’s skyline trading blows with the Oracle.
Clearly, that’s much better than what had been, up until the roller coaster incident, just a taut murder mystery with well-realized characters and eerie supernatural undertones.