Ralph Bailey, bland, intellectualizing professor, is taking his morning walk on the Santa Barbara beach when he finds a curious thing washed up in the surf. The size of a flashlight, with floppy little wings, the creature announces in a tiny, buzzing voice that it sees, but does not hear. It claims to be indestructible and to always tell the truth. Out of pity, Ralph takes the creature home to his wife Barbara and his sons. Then the mischief begins.
The creature, which the Bailey boys have named "Poor Thing" (PT for short) proceeds to spread suspicion, friction, and wounding words throughout the family. They try clever ways to get rid of the creature, like mailing it to Papua New Guinea and laying it on the tracks of an oncoming train, but nothing works. There is no escaping the tiny voice that relentlessly calls truth (and scandal) on everyone.

Having broken through into new levels of honesty with one another, the Baileys give up on ridding themselves of PT. With a curious budding affection for it, the family decides on adoption. PT is accepted into the family, but with strict rules that can easily lead to his being shut away in the refrigerator if he can't behave and restrain his "gotcha" urges and his love of dirt.