Earth has been taken over by a benign group of aliens known as the Monitors, gentlemanly figures clad in black overcoats and bowler hats.

They are dedicated to suppressing humanity's propensities for violence, sex, war, and trouble, enforcing their ethos with spray cans of a pacifying gas and with television ads praising the Monitors' rule—the latter featuring cameos by a variety of comedic actors, as well as bandleader Xavier Cugat and Illinois senator Everett Dirksen (who died before the film's release).

A conflict with the Monitors, inspired by the outrageous antics of a street preacher (Larry Storch), leads to the flight of movie actress Barbara (Susan Oliver), who is a collaborator with the Monitors, along with free-lance pilot Harry (Guy Stockwell) and Harry's brother Max (Avery Schreiber), and their spiriting away by the "preacher", who turns out to be a leader of SCRAG or "Secret Counter Retaliatorial Group", an anti-Monitor resistance group.

Even though he works to overthrow the Monitors, he also comes to oppose SCRAG, the right-wing "citizen's army" that wants to impose its own version of American government.

After Harry learns that he cannot disable the timing mechanism of the weapon, he impulsively decides to try to use it in an attempt to coerce the Monitors into leaving Earth.

The implacable refusal of the Monitors to leave by force ultimately causes Harry to respect their efforts to bring peace to warlike Earth.

Although to the very end Barbara wants no harm to come to the Monitors, she eventually becomes conflicted and disillusioned by their difficulty in understanding complex human emotions like love.

He also serves as a wise counselor to Barbara, even though by the story's end he finally admits he cannot understand the subtleties of human emotions.

He then shows Harry a Monitors' propaganda video that lays bare the ills that beset the pre-world war United States: racism, police brutality, pollution, escapism, and imperialism among them.

Instead, Jeterax states, the Monitors are choosing to leave Earth willingly rather than exert brute physical force to control the world.

Gen. Blackwish (Keenan Wynn) – The leader of SCRAG, the general has a compound just outside Chicago from which he plans to destroy the local headquarters of the Monitors with the implosion bomb "and threaten the rest".

After the Monitors have left Earth, however, the general becomes a top military leader when the president regains power over the United States.