It's a sort of sci-fi creature-feature. In this case the 'creature' is a 'winged serpent', sort of like a flying lizard, that sets up a roost - and lays an egg - in the tower of New York's iconic Chrysler Building. From there it goes on hunting expeditions - apparently the heads of hapless window cleaners, topless rooftop sunbathers, young men in rooftop swimming pools etc are considered to be delicacies.
There is also a confusing subplot about human sacrifice bringing back the ancient Aztec god Quetzalcoatl (Q for short, hence the film's title). David Carradine (of 'Kung Fu' fame) stars as a detective taxed with getting the beast, but only Michael Moriarty (who plays a low-life getaway driver) knows where it roosts.
It's all very silly and to call it a 'horror' movie would be wrong - it's a cheesy period piece, with the odd spot of blood and bare chest on show. In fact it's one of those films that could be classified as 'so bad it's good' and for that reason I'll give it 4/5 stars.