A scientist is working for the good of mankind, trying to grow animals to jumbo size, so, you know, they can feed more people.  Makes no sense, but he experiments with a tarantula (why? nobody is going to want to eat a jumbo spider), and it escapes from his lab much to the consternation of the local town folk.  They try to blow it up with dynamite, but it’s not enough.  They bring in fighter jets & Clint Eastwood and eventually fry the poor thing with napalm.  The movie poster is pretty racy: