Lysergic acid diethylamide - more commonly known as LSD - is one of the most potent pharmacological substances known to science. It is also the most controversial. One ten-thousandth of a gram, an amount invisible to the naked eye, can precipitate some of the most powerful experiences in the repertory of human consciousness. Tonight Everyman traces the turbulent history of this remarkable substance. Are psychedelic drugs merely dangerous and subversive poisons, or do they tell us something important about reality, consciousness and the relationship between them? Among those appearing are Dr Albert Hofmann , the man who discovered the drug in 1943; Humphry Osmond , who coined the term 'psychedelic' and administered mescaline to Aldous Huxley ; and Lord Christopher Mayhew who, 30 years ago, took part in a filmed experiment with a hallucinogenic drug, shown tonight for the first time.