The book outlines contact between Earth and the titular Visitors, a group of mysterious objects from deep space. Visitors are simple black oblong boxes, large as buildings, which take up orbit round the Earth before descending to the USA. Their nature remains mysterious. It's unclear if they're vehicles or living things. They're apparently unable to communicate with humans meaningfully. On one occasion someone is taken inside a Visitor, only to be released reporting experiencing a jumble of confusing colored lights and sounds.
Visitors are composed largely of a dense form of cellulose and proceed to consume plantlife. Eventually they start producing vehicles, superficially resembling cars but capable of flying using the same unknown principles as the Visitors themselves, apparently incorporating some element of intelligence, or at least instinct, since they don't crash into anything.
Humans assume the Visitors have created these vehicles as a gift in return for the plants consumed. The novel touches on the disruption such well-meaning gifts might have to economic systems. Toward the end the Visitors start producing housing units. It's implied that something living may be inside - maybe Visitor-produced humans.