Max Vigne, the most junior member of a survey group mapping the Himalayas in the 1860s, writes letters to his young wife Clara in England. She has prepared in advance of his journey a series of postdated letters which he keeps in his trunk. When these have been read, Clara sends numbered letter packets which arrive sporadically, out of sequence, if at all, over the months of the expedition. Max struggles to describe and to edit his daily experiences on the mountains which are extraordinary, often terrifying, and disorienting for him.

Separated by time, distance, and experiences, they are slowly and irrevocably estranged. Max discovers that his real scientific passion is alpine botany, and he must decide how to tell Clara that he will not be returning to England after the Survey ends. The exchange of letters ingeniously maps out the complexities between Max's love for his wife and his passion for scientific knowledge, and the wide expanse between them.