When the fire engine comes to a stop in the "Burning Bright" section of Fahrenheit 451, it doesn't take long for Montag realize they are in front of his house. He also quickly figures out, as he watches Mildred exit the house, that his wife called in the alarm. Captain Beatty orders Montag to burn down the house and tells him that when he's finished, he's under arrest.
At this point Faber, who is listening through a "seashell" ear piece from his home to Montag's conversation, shouts to Montag: "Montag, get out of there!" Montag hears him, but before he can react:
Beatty struck him a blow on the head that sent him reeling back. The green bullet in which Faber's voice whispered and cried, fell to the sidewalk. Beatty snatched it up, grinning. He held it half in, half out of his ear.
Montag heard the distant voice calling, "Montag, you all right?"
Beatty switched the green bullet off and thrust it in his pocket. "Well--so there's more here than I thought.... We'll trace this and drop it on your friend."
Captain Beatty now knows that Montag is not working alone, and it is his threat to trace the seashell and find the person on the other end that prompts Montag to do what he does next. In order to protect Beatty, Montag points the flame-thrower at Beatty and says to him, accusingly, "We never burned right"
Beatty responds to this by fixing a smile on his face and saying, "Hand it over, Guy." But Montag doesn't obey. Instead, he pulls the trigger:
And then (Beatty) was a shrieking blaze, a jumping, sprawling, gibbering mannikin, no longer human or known, all writhing flame on the lawn as Montag shot one continuous pulse of liquid fire on him.... Beatty flopped over and over and over, and at last twisted in on himself like a charred wax doll and lay silent.
There is now no turning back for Montag, and he takes off running, with the mechanical hound close behind.