What is the main conflict in The Day of the Triffids by John Wyndham?

This novel of speculative fiction, written in the early-1950s, tells a story common to the genre of that era: traditional western ways and philosophies of life are decimated, and threatened with complete destruction, by a combination of mysterious events and the increasing control of a disguised, subversive, murderous entity; in this case, mobile and intelligent plants. As it chronicles the struggles of a small, desperate group of human survivors, the narrative explores themes relating to the conflict between morality and necessity, the nature of human frailty, and the social nature of being human.