Some Do Not … begins with the two young friends, Christopher Tietjens and Vincent Macmaster, on the train to Rye for a golfing weekend in the country.
Both men work in London as government statisticians; though Macmaster aspires to be a critic, and has just written a short book on Dante Gabriel Rossetti.
The second chapter switches to his elegant socialite wife, Sylvia, who is staying with her mother at Lobscheid, a quiet German resort, with their priest, Father Consett.
Valentine is also present, helping Mrs Duchemin, who is apprehensive about her husband because he is prone to fits of lunacy.
He becomes paranoid that the two guests are doctors coming to take him to an asylum, and destroys the decorum of the occasion, ranting about sex first in Latin, then starting to describe his wedding night.
When they're overtaken by Mrs Wannop on her dog-cart, he notices that the horse's strap is about to break, and potentially saves her life by fixing it.
On the way back, Tietjens and Valentine are alone, conversing and arguing in the moonlight, and falling in love, until, in the dawn mist, General Campion crashes his car into them and injures the horse.
He sees Mrs Wannop regularly – she has moved to London, near his office – and has been helping her write propaganda articles.
His nephew, Brownlie, who is infatuated with Sylvia, has unfairly and humiliatingly dishonoured Tietjens' cheques to his army Mess and his club.
The two brothers walk from Gray's Inn to Whitehall, speaking candidly, as Christopher disabuses Mark about the rumours defaming him and Valentine.
He leaves her to talk to Mark while he goes in for a meeting, at which he is offered the chance to be given a posting at home rather than be returned to France for active service, but he refuses.
Valentine recalls a rare conversation she had with Christopher five or six weeks earlier, at one of Macmaster's parties, during which she realizes he will return to the war.
The novel ends with Christopher returning to his dark flat, recalling the events and non-events of the day and night, including his farewell to Macmaster, and especially his last conversations with Valentine.
The novel ends with a short paragraph stating that Tietjens got a lift back towards his flat in a transport lorry.
Two fragments survive of a cancelled ending, including a scene with Sylvia, who has been waiting for Christopher in their flat, hoping to seduce him after he has slept with Valentine.
The 1948 Penguin text repeated the error, which was first caught for the 1950 Knopf omnibus edition of Parade's End.