Naomi Mitchison, daughter of a distinguished scientist, sister of geneticist J B S Haldane, was always interested in the sciences, especially genetics. Her novels did not tend to demonstrate this, and she did not publish a Science Fiction novel until almost forty years into her fiction-writing career.

Isobel Murray's introduction here argues that it is by no means 'pure' Science Fiction: the success of the novel depends not only on the extraordinarily variety of life forms its heroine encounters and attempts to communicate with on different worlds: she is also a very credible human, or Terran, with recognisibly human emotions and a dramatic emotional life. This novel works effectively for readers who usually eschew the genre and prefer more traditional narratives.

Explorers like Mary are an elite class who consider curiosity to be Terrans' supreme gift, and in the novel she more than once takes risks that may destroy her life. Her voice, as she records her adventures and experiments, is individual, attractive and memorable.