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A teenager meets a scientist and his daughter, and discovers that they can travel in time. In the book, time travel into the past can create overlapping branches, potentially leading towards chaos. An evil (or just self-serving and corrupt) version of the scientist and his daughter show up, and tempt the young man into working for them.
Part of the plot involves a hideout far in the past, in the arctic, where there's no chance of affecting causality. The young man is abandoned there for awhile, then rescued... There's a nightmarish scene involving elevators stopping on multiple floors at the same time, and a sequence where the young man sees the scientist doing an elaborate exercise routine with leaps and hurdles, that turns out to be practice for a heist of some kind.