The story begins in the present day in Africa. Harry and Helen are on a safari. Harry is dying. His flashbacks are written in italics. They could be daydreams, but given his physical condition, they might be dreams as he's going in and out of consciousness. In addition to the present-day dialogue and the flashbacks, the third-person narrator adds commentary about Harry's thoughts.
Harry is distraught because he is dying and feels he has wasted his life. The flashbacks and commentary from the narrator reveal his biggest regrets. From the narrator, we learn that Harry does not love Helen. He had been a successful writer earlier in life. When he partnered with Helen, a major factor was her money. Harry blames her for giving him a cushy lifestyle, thus destroying his edge and his passion for writing. It is implied that this is not the first time Harry has used a woman in this way. In fact, using women for their money became his career when he could no longer write: "And he had chosen to make his living with something else instead of a pen or a pencil."
In other words, Harry had sold out long ago. Harry is full of regrets about what he did not get to write: namely, his time traveling in Europe and hunting. (We get all of this from the flashbacks and from the narrator.) And as he's dying, he is taking these frustrations out on Helen.
Henry's angst centers on his regret that he did not write enough. This is from a flashback where he's reminiscing about traveling in Europe and his quarrels with women from his life.
There was so much to write. He had seen the world change; not just the events; although he had seen many of them and had watched the people, but he had seen the subtler change and he could remember how the people were at different times. He had been in it. and he had watched it and it was his duty to write of it; but now he never would.
With only the present-day dialogue, the reader gets the sense of a frustrated, dying man who hates his wife. The flashbacks give the reader the source of his real frustration: that he sacrificed his career for an easy lifestyle with rich women.