Within less than three minutes, I, Frankenstein dispenses with the entire original Frankenstein story, leaving Victor Frankenstein dead of exposure and his monster (Aaron Eckhart) still alive, thanks to his apparently indestructible constitution. The monster returns to bury Frankenstein on his family's estate, and that's where the story really begins: Despite its title, this isn't really a movie about Frankenstein and the creation of his monster, but about an ancient war between demons and gargoyles, in which the monster (who takes the name Adam) ends up caught in the middle. See, the gargoyles (who pose as the stone figures on the sides of buildings, but are also immortal human-looking creatures descended from angels) protect the world from the demons, who are disguised as humans. Because Adam was brought to life by science, he is a person without a soul, and thus somehow vitally important to both sides. So the demons attack him, the gargoyles rescue him and arm him with mystical weapons, and then he rejects both of them to live alone in the wilderness.
It somehow takes 200-plus years for them to track him down again, at which point it is now the present day and Adam has a stylish short haircut and a hoodie to wear under his trenchcoat. He heads to the city where all the main gargoyles and demons conveniently live, where he faces off against the demon prince Naberius (Bill Nighy, the only person in this movie having a good time), who is trying to use the journals of Victor Frankenstein to resurrect thousands of corpses that can be inhabited by the spirits of slain demons to become an army and take over the world. Believe me, none of it is any more coherent than my explanation of it. Not surprisingly, this mashup mythology comes from actor/writer Kevin Grevioux, co-creator of the Underworld series, to which this movie bears a strong and unfortunate resemblance. The hulking, deep-voiced Grevioux plays a supporting role as one of Naberius' demon henchmen, which is a far more enjoyable contribution to the movie than his story.
Theoretically, Grevioux's basic idea (based on a "graphic novel" that as far as I can tell was never published) has been polished by professional screenwriter Stuart Beattie, who has the sole credit for the screenplay and also directed. Beattie's written at least one great movie (Collateral) and also contributed to the Pirates of the Caribbean, G.I. Joe and Punisher franchises, but he can't make sense of this ridiculous mess, and his chaotic direction doesn't help, either. The entire movie takes place in dank, dark spaces, making it look like it was coated in a layer of grime. Beattie's approach to shooting action sequences involves lots of slow motion, and Eckhart doesn't exactly make for an imposing action hero.
The idea of making Frankenstein's monster into an action hero at all is pretty ridiculous, and nothing in the movie makes a convincing case for it. With some artfully placed scars over his buff body, Adam doesn't look particularly monstrous, nor is his angst particularly deep or affecting. Eckhart is severely miscast, and aside from Nighy (who manages to nearly pull off some atrocious lines), no one else in the movie seems to have any clue what they're doing. Yvonne Strahovski plays a sympathetic scientist who helps Adam, and Miranda Otto plays the queen of the gargoyles (!!), but their talents are no match for the incoherent script and the sometimes hilariously florid dialogue. They aren't helped by the special effects, which are ugly and plastic-looking (especially the gargoyle forms of the gargoyle characters, which should at least sort of resemble the actors who play them).
Perhaps the saddest thing about this movie is that it was clearly meant to be the start of a franchise, with Adam giving a closing voiceover about how he will be out there from now on, ready to take on new threats to humanity. Grevioux even planned to have it cross over with the successful Underworld movies (he initially planned for Underworld references throughout the movie, and a post-credits cameo from Kate Beckinsale as her Underworld character), which really aren't much better. They are, however, box office hits, and thanks to the meager returns for this movie, Adam will probably never meet up with the vampires and the lycans. All things considered, that's probably best for everyone.