McCay and his daughter, "pestered to death by mosquitoes" at their summer home in New Jersey, find a professor who speaks the insects' language. The professor tells McCay to "make a series of drawings to illustrate just how the insect does its deadly work", and after months of work McCay invites the professor to watch the film.*
A man looks around apprehensively before entering his room to go to sleep. A giant mosquito (with top hat and briefcase) flies in through a transom after it finds itself too large to squeeze in through a keyhole. It repeatedly feeds on the sleeping man, who tries in vain to shoo away his assailant. Eventually, the mosquito drinks itself so full that it explodes.
* This opening Prologue is presumed lost.