Oh, the vegetable horror! Creepy as they are, the ambulatory triffids are the sort of secondary apocalypse here, with their stinging predation only really able to become a widespread threat when humanity has managed to handicap itself by general blindness and dodgy orbital weapons. Yet for all that rather over the top summary, The Day of the Triffids is essentially a relatively quiet story about a changed world, and how changes in one species open the way for others. It's power relationships and ecology, and a low-key debate over the most civilised way to both adapt to a new world, and to rebuild the best of the old.