Even though Montag and Millie have been married for years, Montag realizes, after the overdose incident, that he doesn't really know much about his wife at all. He can't remember when or where he first met her. In fact, all that he does know about his wife is that she is interested only in her "family" — the illusory images on her three-wall TV — and the fact that she drives their car with high-speed abandon. He realizes that their life together is meaningless and purposeless. They don't love each other; in fact, they probably don't love anything, except perhaps burning (Montag) and living secondhand through an imaginary family (Millie).
When Montag returns to work the next day, he touches the Mechanical Hound and hears a growl. The Mechanical Hound is best described as a device of terror, a machine that is perversely similar to a trained killer dog but has been improved by refined technology, which allows it to inexorably track down and capture criminals by stunning them with a tranquilizer. Montag fears that the dog can sense his growing unhappiness. He also fears that the Hound somehow knows that he's confiscated some books during one of his raids.
The fire chief, Captain Beatty also senses Montag's unhappiness. Upon entering the upper level of the firehouse, Montag questions whether the Mechanical Hound can think. Beatty, who functions as the apologist of the dystopia, points out that the Hound "doesn't think anything we don't want it to think." Instantly, Beatty is suspicious of this sudden curiosity in Montag and questions whether Montag feels guilty about something.
