This story is the continuation of The Gunslinger, Roland of Gilead, and his quest towards the Dark Tower. The book begins with Roland waking up on a beach where he eventually encounters three doors. Each door opens onto New York City at different periods in time (1987, 1977, and 1964) and as Roland passes though these doors he brings back the companions who will join him on his quest to the Dark Tower.
The first door brings Eddie Dean, a heroin addict. The second door finds Odetta Holmes, a rich, black woman in a wheelchair from a subway accident. Odetta also has a violent split personality named Detta Walker which she is completely unaware of. The third door that Roland encounters doesn't bring a new companion, but rather brings Odetta and Detta together forming a third woman in Susannah.
Each of these people are essential for Roland to continue his quest. They are all part of a "ka-tet". Defined as one made from many and sharing the same destiny.