We see immediately why people call Big Sur "dark." Jack wakes up hung over in San Francisco and we know from the start that we're not dealing with the happier 25-year-old Jack from On the Road.
While the initial situation is rife with conflict, we can identify this line as the impetus for the story's basic plot line: a trip (or two, or three) out of the city to Monsanto's cabin in the woods.
Jack is happy with his time alone in Monsanto's cabin. But he never lets his readers forget that all experiences mark preludes to the real meat of his story – his fit of delirium on the night of September 3rd. Every signpost – the death of his cat, of the goldfish, and of the otter, the sickness of George Baso, the vision of the flying saucer, and the night on the beach with Ron Blake –function as speed bumps on the road to the novel's climax.
We've been building towards this night for the entire novel. All of Jack's "signposts," all his "hints of coming madness," and all his hesitations and insistence that "something is wrong" refer to this, his breakdown and delirium on the night he writes as September 3, 1960 (see "Setting" for discussions of exact time and place). As readers, we're certainly not disappointed. Jack delivers an explicit, visceral, wrenching account of his experience that night. This experience is not to be missed by, so make sure you look it up in your text.
After the horror of the delirious hours of the night, we still have to deal with Billie and her staggering instability. Remember that she's threatens to kill herself, her son, or both, several times in the course of the novel. When she digs a hole that is the exact size and shape of a child's coffin, we know we're not out of the woods yet – literally and figuratively.
In the "golden swarming peace of heaven" that is sleep, Jack leaves the torture of the past twelve hours. "All the dark torture is a memory," he writes.
Needless to say, this is a surprisingly optimistic ending to page after page of delirious nightmares in the woods. Kerouac re-interprets even the most painful of events (like his cat's death) in a new and hopeful light. See "What's Up With the Ending?" for a full discussion.