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The Lightning Thief
- The film switches the driving force behind Percy's quest so that he's trying solely to rescue his mother from the Underworld rather than reclaim the bolt from Hades. With a few changes, this doesn't have a major effect on the story until you factor in the escape from the Lotus Casino. In the book, their five-day stay in the casino was significant because it left Percy with only two days to find the bolt and return it to Zeus. The film also tries to include this dilemma, despite it being completely broken by the fact that Percy was never looking for the bolt in the film's plotline; he was in this solely to save his mother and has no idea where the bolt is. So he and his friends stressing over their "deadline" is completely nonsensical, because they never had a deadline to begin with.
- Rather than just having Percy come across the guy from the 70s on his own like in the book, in the film, Poseidon has to warn him telepathically about being trapped inside the casino before he thinks to investigate anything. But he still only does this after Percy has been inside the casino for five days, begging the question of why he waited so long when time was of such a crucial essence.
- The film also has Persephone in the Underworld at the time the heroes visit there, which is on the eve of the summer solstice when she should be in the world above, like she was in the book. Even stranger is that she acknowledges that she does have allotted time to spend away from the Underworld, but whether the allotment has changed since the time of the myths or Hades was somehow keeping her trapped there against her will isn't addressed.
Sea of Monsters
- The second movie can't decide on which kinds of monsters can pass through the barrier and which ones can't. In the book, it was made explicit that no monsters could access camp unless a camper gave them verbal permission, as Annabeth had to do for Tyson, and the bronze bulls that attacked the camp were fought on the hillside outside the barrier as they were obviously not allowed in. The film adds in a caveat that Tyson could enter camp on his own, explaining it away as because he's a son of Poseidon, despite that being the case for all cyclopes (a fact the film acknowledges), including the ones who killed Thalia and were shown being repelled by the barrier.
- Towards the end, the movie flip-flops on whether the story will have or need any further continuation after its conclusion. The early revival of Kronos means the terms of the prophecy have technically already been fulfilled, the fact that he swallowed Luke whole would seem to ruin any desire to try resurrecting him again, even if he manages to escape being eaten by Polyphemus...and yet after all of that is over, the movie still closes out with Thalia's revival and Percy realizing she could be the half-blood destined to fulfill the prophecy.