The Day of the Triffids (Penguin Twentieth Century Classics)

This book is fantastic and frightening, but entirely plausible. It doesn't just seem scientifically possible, but its characters are living people shaken out of the civilization they know into the horror of a world dominated by triffids.

The triffids are grotesque and dangerous plants, over seven feet tall, originally cultivated for their yield of high-grade oil. So long as conditions give the mastery to their human directors, they are a valuable asset to mankind. But when a sudden universal disaster turns those conditions upside down, then the triffids, seizing their opportunity, become an active and dreadful menace.

The story of what happens is told here by one of the few people lucky enough to escape the disaster.