To summarize briefly: in his early 20s, Terence and some colleagues went to Colombia with the intentions of finding and studying a certain entheogen. After (I think days or weeks) of consuming psilocybin (mushrooms), their investigations became more focused on the mushroom, and their collective ideas about the matter become increasingly "out there." Terence and his brother become convinced by their experiences that extra-dimensional entities are instructing them to conceive of an "experiment" (that involves them taking both psilocybin and ayahuasca) which will trigger some sort of eschatological event. The "experiment" then causes Terence and his brother to have mutually re-enforcing psychosis and hallucinations that last for something like two weeks.
The book is very interesting. Terence was a hell of a writer, and the general concepts touched upon are highly interesting.
From the article: "Does the delusion of one visionary ecstatic validate the delusion of another? How many deluded, or illuminated ecstatics does it take to make a reality?"