As he waits for the train to take him to a weekend party in the country, the narrator notices that Gilbert Long seems much more assured and lively than before. He also sees that Mrs. Brissenden (nicknamed "Mrs. Briss") is much younger-looking than her husband, though she's actually ten years older. The narrator begins to theorize that Long and Mrs. Briss are getting their vitality, vampire-like, from the "sacred fount" of their sexual partners' energy. At first, the narrator theorizes that the source of Long's newfound assurance and intelligence is a certain Lady John.
Later he changes his mind, as he constantly discusses his ideas with others at the party, particularly an artist, Ford Obert. The narrator notices that another woman at the party, May Server, seems listless, and he starts to wonder if she may be the lover providing vitality to Long. Eventually, the narrator begins to construct enormously elaborate theories of who is taking vitality from whom, and whether some people are acting as screens for the real lovers. In a long midnight confrontation with Mrs. Briss which concludes the novel, she says the narrator's theories are ridiculous, and he has completely misread the actual relationships of their fellow guests. She finishes by telling him he's crazy, and that last word leaves the narrator dismayed and overwhelmed.

Question: Why did Mrs Briss become angry in the story? 
Answer: Because she thought some of the elaborate theories about the guests were ridiculous.
Alternative answer: she is irritated by the narrator's outlandish theories, in which acquaintances are draining energy from their lovers