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A robot designed to play a simple game of laser tag with humans (it shoots rubber balls, the people shoot invisible lasers from toy guns) takes the game too seriously and decides that people should be killed. So it switches to using real weapons and starts attacking people for real. And starts using advanced tactics, like electricuting a swimming pool to kill the person in the water.
The robot's progression from harmless-but-annoying toy to evil threat is silly. It starts out playing its game with this family - the only thing it seems to know how to do, actually - and the thing seems hardly a threat with its tendency to verbally publicize its every action, as it slowly wheels around, slowly turns, and then shoots with delayed precision. But when it becomes evil, it starts wielding knives and ball bearings, and then it kills.
The whole thing is absurdly implausible. The only reason this slow-moving, verbally telegraphing robot is such a deadly threat is because every character is carrying the idiot ball. The first time it showed signs of being dangerous or at the very least a nuisance, such as when it shot a mirror with a ball bearing and shattered it into pieces, it should have been taken out of the house; never mind turning it off (which was smart, but didn't work) or removing the battery (also smart, though the little sister of course put it back in to play with the robot). I really have to question how this robot could sneak into a high school, murder a jock, and then return home without anyone noticing.
Besides the unintended comedy, it's also a legitimate comedy. It has its share of genuinely funny jokes, like when the robot picked up on how to use profanity via exposure to TV shows, and even copied the little sister character's childish insults. There's just something funny about seeing this small, slow-moving robot say "Drop the gun or the bitch gets it" in a robotic monotone. It could hardly be less threatening.
The movie may have been a failure in its time, and for good reason: it's really stupid. But at the same time, it delivers what it promises: a robot that turns evil and kills people. It's just that all manner of ridiculous contrivances need to occur in order for that premise to work.
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