Mae January yn blaned sy’n marw – wedi’i rhannu rhwng tywyllwch wedi’i rewi’n barhaol ar y un ochr, a heulwen danbaid ddiddiwedd ar y llall. Mae dynoliaeth yn glynu wrth fywyd, wedi’i gwasgaru ar draws dwy ddinas hynafol a adeiladwyd yn y mymryn o gyfnos sy’n goroesadwy. Mae Sophie, myfyriwr a chwyldröwr anfodlon, i fod wedi marw ar ôl cael ei alltudio i’r nos. Gan oroesi drwy ffurfio cysylltiad anarferol gyda’r bwystfilod sy’n crwydro’r rhew, mae Sophie’n addunedu i aros yn guddiedig o’r byd. Ond mae gan ffawd gynlluniau eraill – a bydd siwrnai Sophie a’r teulu gwerinos sy’n ei dilyn yn newid y byd i gyd.

“If you control our sleep, then you can own our dreams… And from there, it’s easy to control our entire lives.” January is a dying planet – divided between a permanently frozen darkness on one side, and blazing endless sunshine on the other. Humanity clings to life, spread across two archaic cities built in the sliver of habitable dusk. But life inside the cities is just as dangerous as the wastelands outside. Sophie, a student and reluctant revolutionary, is supposed to be dead, after being exiled into the night. Saved only by forming an unusual bond with the enigmatic beasts who roam the ice, Sophie vows to stay hidden from the world, hoping she can heal. But fate has other plans – and Sophie’s ensuing odyssey and the ragtag family she finds will change the entire world.