In the four weeks before his army contract runs out, Tristram spends long hours at the Central Library, "rehabilitating" his skills as a teacher.
When the month is out, he heads to the War Office in London. There, he reports as "the sole survivor of one of those pleasantly contrived miniature massacres on the West Coast of Ireland" (6.2.8), and informs the Army Major in charge that his contract is officially terminated.
The Army Major is surprised to see that Tristram is alive, and fills in the pieces of the story that Tristram hasn't already figured out for himself. What's clear is that the British Army is using the fake battles—Extermination Sessions, they call them—to cull the surplus population, and, at the same time, to produce more meat for the canning factories. Chilling.
The Army Major signs Tristram's discharge papers, and lets him go.
Tristram takes the tube-train to Brighton, and finds himself an apartment and a new teaching position.
Having organized all of his affairs, he then heads towards the sea, where he knows he'll eventually find Beatrice-Joanna out for an afternoon walk with the twins.