The first part of the plot, which occurs in Chapter I, is the meeting between John Marcher and May Bartram on an October afternoon. Though Marcher doesn't really remember it, they met ten years earlier in Italy, when he confided in her that something special was going to happen to him. She recalls the following:
You said you had had from your earliest time, as the deepest thing within you, the sense of being kept for something rare and strange, possibly prodigious and terrible, that was sooner or later to happen to you, that you had in your bones the foreboding and the conviction of, and that would perhaps overwhelm you.
In other words, he told her that he had a presentiment that he was going to meet with a great fate, but he didn't know what this fate would be. Over the years, she often thought of his confession. They agree to wait together for this fate, as she shares an interest in his strange obsession that a great event is going to befall him.
The second part of the plot involves their waiting for this mysterious fate to happen to Marcher as they pass the time rather innocently. May is able to purchase a house in London, and Marcher, while polite to her, remains aloof and distant in some ways. He believes that "something or other lay in wait for him, amid the twists and the turns of the months and the years, like a crouching Beast in the Jungle." In waiting for this mysterious beast, he becomes entirely self-obsessed. In their relationship, she is described as "his kind wise keeper, unremunerated but fairly amused." In truth, she gets very little from this strange relationship as the years go by and Marcher waits for the metaphorical Beast in the Jungle to spring at him.
In the third part of the story, they go on like this for some time, with her living only to make him seem more ordinary and passable to the outside world. May then learns that she has a disorder of the blood. She assures Marcher that his fate is still special, but that it is never too late to escape it. However, he doesn't understand her meaning, and she dies. He still wanders on in a senseless and uncomprehending manner that James describes in the following way:
What it presently came to in truth was that poor Marcher waded through his beaten grass, where no life stirred, where no breath sounded, where no evil eye seemed to gleam from a possible lair, very much as if vaguely looking for the Beast, and still more as if acutely missing it.
Marcher continues to look for the beast, but it evades him. As a result, Marcher has a dull and dry existence.
In the last part of the plot, Marcher travels around the world and then returns to see May's grave. As he is standing before it, he sees a mourner struck with grief before another grave. In looking at the stranger's face, he realizes that his Beast in the Jungle has been to live without passion. As James writes, "The fate he had been marked for he had met with a vengeance." In the end, Marcher realizes that he has met the fate he once feared, but he was too stupid to realize it earlier.