This is going to be one of those books that is really hard to talk about. Not because I'm worried about giving away spoilers, but rather because I'm not sure how much I understand it.
It starts well. A group with a common interest in witnessing the supernatural come together to observe a "summoning", that goes well until interrupted by a rude stranger. One of these observers (Maskull) is then invited by the stranger to visit Arcturus, a planet in a distant binary star system. Unbelieving at first, he is eventually convinced to see go along with him. The voyage begins from a remote observatory in Scotland and Muskall is put to sleep for the journey. He awakes on Arcturus abandoned by his companions and finds himself now in possession of an alien body with strange appendages and new sense organs.
Things then get stranger by the page as he Maskull journeys across this new planet, drawn northwards and encountering an array of alien beings who briefly accompany him on his travels. The importance of his physical journey gradually begins to dwindle in the wake of his spiritual, philosophical and moral journey.
Towards the end, the essential nature of existence is unveiled turning any preconceptions we might have had until that point on their head. That is, preconceptions I might have had had I not read the spoiler ridden introduction first.
All in all it was fairly hard work but should appeal to those that like reading a narrative that consists mainly of philosophical musings and descriptions of deeply imaginative environments.