You may have guessed already that Nathan himself has ulterior motives and other secrets, which of course he does, and which of course reveal themselves in the course of the story, but the bulk of this film revolves around the conversations between Caleb and Ava, and Caleb and Nathan, involving why and for what purpose Ava has been built, and how she functions. Not technically, that would be too "textbook" and dry for the audience (as Nathan literally says at one sly moment in the movie), but more in the vein of how Ava makes Caleb, as a straight, hot blooded young male, feel about her. Ava herself is a striking cinematic creation (maybe the only one in this movie), with just Alicia Vikander's face displayed on a part glass, part wired body with unfinished limbs and no skin besides the hands and head. She doesn't act fully human, with all the physical tics and unemotional vocal tones of the android she's supposed to be, but I suppose with Vikander's human face, that's all it takes for Caleb to find himself falling for her in spite of himself.
Despite the interesting visual image of Ava, Oscar Isaac is the most charismatic presence in the movie, elevating Nathan to seem not quite mad, just arrogant and possessing the god complex that anyone who invented the world's most powerful search engine at 13 years old probably would be. A fine actor who's shown himself capable of carrying films with a powerful and commanding screen presence, this is yet another impressive example after Inside Llewyn Davis and A Most Violent Year. But the movie itself is a cold, rather distant rendering of a familiar story (your too successful invention gone rogue), with just enough interest demanded by Isaac and commanded by the visual power of Ava, that it passes for a more thought provoking sci-fi than you'd find in a typical robot adventure. Just don't go in expecting too many fireworks.
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