The Night the Heads Came is a thriller that puts an emphasis on action and does not have much thematic depth or sophisticated characterization. The main character is the narrator Leo Kasden. He and his friend Tim are soon to be seniors in high school. Tim is an artist with hopes of becoming an illustrator of books; it is his ambition that gets the story moving and provides much of the impetus for events as the plot unfolds.

Leo has a few traits that prove invaluable to the narrative. He is a man of action who doesn't fall into confusion and panic when faced with new problems; he confronts them directly and quickly devises plans of action. This intrepidity and resolve allow him to be not only an observing narrator but a fully active participant in the forefront of events where the most intriguing and crucial action takes place. Leo is capable of far more than just physical action. He has an active intelligence, and his quick and nimble mind sorts evidence to decide what he must do to thwart the plans of hostile aliens. He also has a gift for description that compares well with Tim's gift for illustration.

The startling stars of the novel are the aliens from other worlds who are fighting a skirmish in their cosmic war, and Leo's descriptions capture their strangeness, as in this passage about the workers for the heads:

The creatures are tall, taller than Tim and me, with two long, pale arms that seem boneless, like tentacles. I can tell they are very thin, even though they are wearing loose, sleeveless robes that hang from their long necks to the floor. The truly horrible thing about them is their heads, simply because they are so tiny in relation to their height, about the size of tennis balls. The heads are smooth and gray and almost featureless, with one lidless eye in the front and another in the back, and underneath the eyes a mouth like a line without lips that seems to go all the way around the head. In fact, looking at their heads and the way they move, it becomes apparent that these creatures don't have a front or a back; they are the same on both sides, gliding backward or forward without having to turn around.

These efficient beings are the first aliens seen by Leo and Tim, but they are merely the functional bodies of even weirder beings, the heads:

They are shorter than the ones that brought us here and a whole lot uglier, because they are not even remotely humanoid. They basically consist of big, squashy heads, about three feet in diameter. Where the ears would be there are instead appendages like hands with three fat, blunt fingers. I don't even know if they have feet because the soft flesh of the heads lies in folds on the floor. They move by kind of oozing along, like slugs.

These are the beings who try to manipulate Leo and Tim in their effort to influence the behavior of the human race. Although they seem to discard Leo while taking Tim away on a journey across the galaxy, they have actually concocted a scheme in which Leo's penchant for taking action plays an important role.

Tim is the stereotypical self-absorbed artist who is so rapt in his art that he fails to even think about the events surrounding him. So far as he is concerned, the heads kidnaped him in order to train him to be a great artist: "The heads. They loved my drawings. That's why they kept me. They decided they wanted to make me into a really great artist." When he reappears, having lived two years with the heads even though only days have passed on Earth (the result of the time dilation effect predicted by Einstein), he is enthusiastic about the new kinds of illustrating that he has learned, but it is Leo who realizes that Tim is being used for a purpose other than creating great art. Tim's illustrations do depict scenes of beauty, but they also show horrible scenes that the heads told him to study and then illustrate. Tim has learned to create a compelling three-dimensional effect for his drawings, images that if people look at just right become vividly lifelike. As Leo explains:

But I keep looking. And as I do, the pictures begin to emerge—pictures that are all the more realistic and three-dimensional because they do consist of so many complex lines and cross-hatchings. I see several views of a rough-hewn city; all the buildings are carved out of translucent gems of various colors. It is a mountainous, vertical city designed for creatures who can fly: There are entrances to the buildings at all levels and no stairways or ramps or elevators. The bodies of the birdlike creatures flocking among the buildings are very small in relation to their wings, but their bald heads are quite large.

Their faces are streaked with blood. They are all eating what look like living humanoid creatures and dropping gobs of flesh to the ground. The countryside around the city is barren and arid, and the air is filled with smoke.

There are a variety of drawings of places where the environment has been polluted beyond repair: lands of desolate emptiness, worlds of poisoned life, and cities decayed and populated by horrible, degenerate beings. These pictures are meant to show humanity what has happened to other worlds that abused their environments beyond repair. The heads do not do this out of concern for humans; in fact, they do not much care for them because "humans are dangerous, ruled by greed and emotion."They do what they do to thwart the Others, foul beings who thrive on the destruction of the environment. When Leo attempts to rescue Tim from the Others, he gets a close look at them:

The things squat around Tim like toads as big as large dogs, but with no skin on their bodies, so that the yellow muscle tissue and the purple veins are exposed; their webbed hands and feet are splayed on the floor. The toad like, skinless aspect of their appearance in itself wouldn't be so bad after all, I got used to the heads, who aren't exactly pretty—but the really disgusting, unspeakable part is the faces, the human faces, that stare directly up from the backs of their hunched-over bodies. The faces are about twice the size of human ones and horribly stretched out and deformed; the eyes stare blankly and pointlessly up at the ceiling.

These are the enemies of the heads, and they have come to our planet to push it beyond the brink of environmental catastrophe; if they succeed all Earthly life will be devastated. According to the heads, "The Others are beings that hunger for-planets approaching environmental collapse."The Others turn out to be responsible for the numerous weird stories of abduction by aliens from outer space. They have done so by replacing the truth about alien abductions with false memories, as Dr. Viridian does with Leo. As Leo explains, "What I am saying is that I think the doctors took away our real memories and gave us these other stories. They did it so that no one would believe us and so that no one—including us—would know what really did happen to us." The Others are shape-changers who masquerade as humans in order to deceive us.

The Others did to the world of the heads what they are trying to do to Earth, and the heads take revenge by warning other planets of the evil the Others intend. The heads see in Tim and Leo the potential for alerting humanity to the threat they face from the Others. Once warned, the Earthlings can save their planet and deny another world to the marauding Others. The heads at first underestimate Leo, who proves too independent-minded to be moved by their initial attempts to manipulate him. Leo quickly realizes that the heads have ulterior motives in their dealings with Tim, suspecting that "He [Tim] really must be brainwashed." Leo's resistance to the efforts of the heads to get him to act on their behalf gives him an unusual depth and thoughtfulness for the protagonist of a thriller. His insistence on thinking matters through and not accepting simple answers and solutions shows him to be uncommonly tough-minded. It also serves to constructively complicate the plot. Part of what makes The Night the Heads Came entertaining is how Leo works his way through a plot that becomes ever more complex by facing the toughest questions with uncompromising mental rigor. The heads succeed in manipulating Leo by finding a way that plays to Leo's strength of character: "But you are a person of action, Leo," they note. Leo seems to agree, saying that "I've gotten accustomed to taking control of things, and I like the way it feels." They thus give Leo cause for action, and he unwittingly aids the heads in their war against the Others by defying the heads: "I know for sure now that I can't trust the heads. They lie"; "I see now with absolute clarity that I must not do what the heads are commanding me to do. I know this because of the way the heads lie, the way they don't explain, and the way they threaten." Leo attains a deeper level of thoughtfulness to discover the manipulations of the heads and the complexity of their schemes. This long stride in maturity makes him a formidable opponent for anyone who would try to manipulate or intimidate him again.