After witnessing the landing of a flying saucer, architect David Vincent (Roy Thinnes) warns a skeptical world of aliens' impending takeover, but he winds up alone and on the run when colleagues dismiss him and the invaders give chase. In this ominous sci-fi series, Vincent roams from town to town, seeking aid from an astrophysicist, investigating a butterfly attack, and eluding capture by the aliens, who assume human form and dissolve on death.