A man is drowning in the surf: hands pull him ashore, next to a female swimmer with close-cropped hair. When he wakes again she’s gone, prisoner of the Spine Kings. The man with no name is a solitary stranger in a surreal place, where a single ridge of rock – the Spine – circles a gigantic planet.

Having no other purpose or destination, he sets out to rescue the swimmer. On the way, he encounters strange and fascinating characters: tree-people with shrubs growing from their shoulders, sorcerers who dance every night, women with second heads. The tree people gift him with a name, Thel (meaning ‘treeless’), but they can’t tell him of his origins, or their own.

Gradually it becomes apparent that this isn’t simply a heroic quest with rescue as its goal. Thel and the swimmer (who remains unnamed) meet, part, and meet again as they journey west along the Spine. Thel – who may be a traveller from another world – suspects that the Spine is unnatural. He asks each group of people how the world came to be, and listens gravely to the cosmologies they recount.

The sorcerers are perhaps the most credible. The gods (who ‘fly through space in bubbles of glass’) argued over aesthetics, and whether beauty is an independent quality or if it depends on love and loss. The world of the Spine is their experiment; it has been made as beautiful as possible, while ‘leeching every living thing of love, to see if the beauty would yet remain. And here we are.’

It’s a credible cosmology because Thel constantly regrets the passing of time: each time that he recognises beauty, he is overwhelmed and wishes the moment to last forever. But he’s no native, and he is still capable of love: it is the concept of past (and the expectation of future) that he lacks.

It’s not clear whether the swimmer is a native or not. Near the end of the book, Thel realises that she doesn’t share his language: "when she said arbitrary she meant beautiful, and … when he said ‘I love you’, she thought he was saying ‘I will leave you’". Eventually both are transformed, and their different origins become explicit. Whether the narrative is as circular as the Spine, and that transformation is simply the start of another cycle, is less clear.