Hay Fever takes place in the comfortable, but untidy home of the charmingly eccentric Bliss family. Judith, the mother, is a retired stage actress for whom all the world is, indeed a stage; David, the father, is a novelist. Their grown children, Simon and Sorel, live at home with them. As the play opens, Simon and Sorel are dallying in the living room, exchanging insults about mutual friends and worrying that their mother is "up to" something. They speculate that she has invited some "dreary, infatuated young man" to the house for the weekend.
Sorel is concerned about this possibility because she, herself, has invited Richard Greatham, a proper English diplomat, as her guest for the weekend. Judith comes in from the garden and reveals that she has indeed invited a young man, Sandy Tyrell. Simon adds to the mounting concern and hilarity by mentioning that he too has invited a weekend guest: Myra Arundel. Finally, David comes downstairs from his study and adds one more visitor to the list; Jackie Coryton, a "perfectly sweet flapper."
After more arguments and witty bantering, Judith announces that she has decided to come out of retirement and revive one of her greatest hits, Love's Whirlwind. As she and the children begin to perform on of their favorite scenes from the play, they are interrupted by the arrival of their guests. In their blithely ill-mannered and unconventional way, the family members greet their visitors brusquely and leave them to make acquaintance and fend for themselves.
Following dinner, the Blisses and their guests remove to the living room for a charades-like game of "Adverbs," in which the players attempt to perform an action "in the manner of" an adverb. Not unexpectedly, the hosts enjoy the game more than their guests. For the Blisses the evening is full of sparkling witticisms and clever quips, but for their guests the game is simply too dazzling, and they are quickly overwhelmed by the speed of the sometimes acerbic proceedings.
Once the game is concluded, the family members begin flirtatious liasions with their visitors, but not necessarily with the guest each invited. Judith gets cozy with Richard. Sorel ducks into the library with Sandy. Simon and Jackie take a walk in the garden. And Judith discovers David and Myra together, giving ample opportunity for more dramatics from the aging actress. The evening draws to a rollicking close as Judith, quite carried away and egged on by the family, reprises scenes from Love's Whirlwind, much to the horror and consternation of the four houseguests.
The next morning, Jackie and Sandy reach the obvious conclusion that they are dealing with a family of lunatics, a family that escaped normalcy years ago. The four beleagured visitors band together, decide to depart for London in Sandy's car and hasten to make their escape. Meanwhile, the Blisses gather downstairs for breakfast. David tries to read the final chapter of his novel, The Sinful Woman, to his family but is soon embroiled in an argument with Judith about the geography of Paris. Simon and Sorel cannot help but join the debate, and eventually the whole family is once again at each others' throats.
The guests use this moment of pandemonium to slip quietly away. The oblivious family settles down to enjoy its breakfast, Judith once again announces her plans to return to the stage, and David continues reading The Sinful Woman.