Turned on by a teacher's musings about stepping across the river of time, Zan, fourteen, is further shaken by being mugged and by her brother and his friends reading her diary--all of which puts her in a frame of mind to wish herself, intensely, elsewhere. Transported then to the forest world of a peaceable band of prehistoric cave dwellers, she makes daily attempts to return but eventually learns their language and their ways, exults in her adopted sister's joyous menarch ritual, and shares the common grief when her little ""brother"" dies of a fever. But the society's wise woman has her doubts. . . much ill has come with Zan. . . and in the midst of general uproar and disaster occasioned by the jackknife she brought with her, Zan, in mortal danger, is able to step back across--after an eleven month absence by her time which is only hours later back home. This unpromising fantasy redeems itself at every turn: though Zan gets in the end the first period she'd longed for in the beginning, the wish fulfillment is tempered by the contrast between her mother's passing congratulations and the People's glorious celebration of such an event; though the theoretical background of the time trip is flimsy, the inadequacy (and eventual emotional betrayal) of the teacher who proposes it adds dimension to Zan's lonely accomplishment; and though the opposition of the two cultures, to our disadvantage, is a current cliche, Zan's interim companions and her relationships with them are realized with convincing ease.