Montag is a firefighter. However firefighters in this time period (future) burn books.
The Mechanical Hound lived in the fire house. The Hound had a four inch hollow steel needle in its mouth that would inject massive amounts of morphine or procaine into their prey to kill it.
His boss Beatty often tells stories about why books are illegal to own.
One day there was a call. When Montag and the rest of the firemen raided the house of books and poured kerosene to help light the house on fire to burn the books inside. The older woman who lived there refused to leave and stood on her porch and flicked a match and burned along with the house. This image stuck in Montag's mind.
Montag stole a few books from the old woman's house because he was curious about what was in them. The next day he had a "fever" and called into work sick. He and his wife Mildred spend the afternoon reading. She however prefers to be entertained by the TV walls. Beatty came to his house and told him that every firefighter at one point or another becomes curious about what is inside books and takes one home. He then told Montag that as long as they burn the book within 24 hours nothing would happen.
Montag remembers an afternoon where he saw an old English Professor named Faber in the park. Faber was reading a book and tried to hide the book and run away when he saw Montag, but Montag reassured him that he was safe and they ended up talking. Faber gave Montag his phone number and address. Montag calls Faber and he refuses to talk. Montag goes to Faber's house and shows him the book. Faber is still hesitant to help Montag but eventually agrees to contact someone who used to work at a printing press, in hopes that he will reprint the books Montag has. Faber also gives Montag a two way radio that fits in his ear so he can listen to what Beatty has to say and tell Montag how to react to him.
Mildred had turned her house in. Beatty forces Montag to set his own house on fire and warns him that the Hound is on the watch for him if he tries to escape. When Montag finishes burning his house Beatty places him under arrest and notices that he is listening to something and hits his head. The radio falls out of Montag's ear and Beatty threatens to have it traced to find the person on the other end. Montag turns his flamethrower on Beatty charring him to a crisp. The Mechanical Hound inject's Montag's leg before Montag destroys it with his flamethrower. Montag then takes off on foot and hears a police warning to be on the lookout for him.
Montag ran to Faber's house and took a suit case of dirty clothes in hope the Hound will lose his scent. Faber tells Montag to head towards the river, which will lead him to train tracks where a group of homeless intellects will be. Montag leaves Faber's house and heads towards the river. The Hound is still chasing Montag and everybody is watching the chase on television. In order to stop the Hound from picking up his scent he enters the river and drifts away with the current. He eventually washes ashore and sees tracks, which lead him to a group of people around a fire. The group is waiting for humanity to be ready to read books again. They are memorizing books for that purpose.