On a stormy night, young Jim, who transports a luxury car from Chicago to California to deliver it to its owner, feeling tired and sleepy, picks up a mysterious hitchhiker, who has appeared out of nowhere, thinking that a good conversation will help him not to fall asleep. He will have enough time to deeply regret such an unmeditated decision.

Dr. Pretorius and his colleagues are working on a sensational experiment: by means of stimulation of the pineal gland, they want to open the human mind to higher dimensions. When the experiment succeeds, however, they are immediately attacked by terrible life forms, which apparently are floating around us all the time. When Dr. Pretorius is killed by one of them, Dr. Tillinghast is under suspect and thrown into the psycho ward due to his stories. Only the ambitious psychologist Dr. McMichaels believes him and wants to continue the experiment.

After learning that a boy their age has been accidentally killed near their rural homes, four Oregon boys decide to go see the body. On the way, Gordie, Vern, Chris and Teddy encounter a mean junk man and a marsh full of leeches, as they also learn more about one another and their very different home lives. Just a lark at first, the boys' adventure evolves into a defining event in their lives.

When Seth Brundle makes a huge scientific and technological breakthrough in teleportation, he decides to test it on himself. Unbeknownst to him, a common housefly manages to get inside the device and the two become one.