In the year 2051, Brother David, a Jesuit, and Sister Helen find themselves posted on the Moon's far side, there to do religious observances, but also to conduct astronomical research. Brother David works at Daedalus Station within easy reach of DURT (Distant Universe Radio Telescope), the largest instrument of its kind ever constructed. Sister Helen, a Benedictine, spends her days in a monastery more than a hundred kilometres away on the rim of Icarus Crater.

 The days are long and the nights very dark. David and Helen begin writing to each other, sharing their innermost joys and fears.