The Buick was left at a gas station by an enigmatic owner who immediately disappeared without a trace and was never seen again. As a "lost object", it is then held by police Troop D. How the owner was able to drive the Buick is unknown, as the car appears to humans as unusable: the steering wheel is immobile, the dashboard inoperable, the engine has no moving parts and ignition wires that go nowhere. In addition, the Buick has a regenerative factor, healing off damage inflicted to it.
The Buick's most notable feature is that it acts as a portal to a parallel dimension - not all the time, but apparently whenever it feels like it. This portal goes both ways. It can fling extradimensional items and creatures (all extremely alien in their biology) into our dimension. Usually, creatures coughed up by the Buick into our world quickly die and eventually decompose, though some can survive for a while and pose a substantial threat. Even worse, the Buick can absorb objects and creatures from our dimension in the vicinity and send them into the other, and has occasionally done this to humans.
In addition, the Buick occasionally starts producing pulsing flashes of extremely bright light ("lightquakes"), strong enough to blind unprotected eyes. Those lightshows are associated with all electronic items in the vicinity going dead. The abilities of the Buick are extremely erratic and it is hard to predict when it is just a very strange car and when it is a hazardous portal. The mechanisms of the portal are also poorly understood. At one point, it has snatched a gerbil from a closed cage (without damaging the cage), but another time a simple rope stopped it from pulling a human in.
The Buick is hinted to be at least somewhat sentient and malevolent. It seems to be able to implant suggestions into nearby minds in order to lure them close and snatch them into the other dimension (this seems to have happened to Ned Wilcox, who attempted to destroy the automobile in a brainwashed trance and was nearly absorbed into the parallel dimension, getting saved just in time by members of Troop D).
It has also been theorized that destroying the Buick may result in the dimensional portal going completely out of control and causing massive damage to our world.
End of the Buick
At the end of the novel, the Buick's window cracks and remains cracked, its regeneration not kicking in. It is implied it expanded the last of its energy and is at the end of its rope, though we don't get to see its ultimate destruction.