Hmm..where to begin...
It tells the story of the gunslinger, Roland of Gilead, and his quest to catch the man in black, the first of many steps towards his ultimate destination - the Dark Tower.
The main story takes place in a world that is somewhat recognizable as the Old West but exists in an alternate time frame or parallel universe to ours. Roland exists in a place where "the world has moved on." This world has a few things in common with our own, however, including memories of the song "Hey Jude" and the child's rhyme that begins "Beans, beans, the musical fruit." Vestiges of forgotten or skewed versions of real-world technology also appear, such as a reference to a gas pump in a tunnel under a mountain that is worshipped as a god named "Amoco", and an abandoned way station with a water pump which is powered by an "atomic slug".
As Roland travels across the desert with his mule in search of the man in black, he encounters Brown, a farmer and Zoltan, his crow, who graciously offers to put him up for the night. While he is there, we learn of his time spent in Tull through a flashback. Tull is a small town which Roland came to during his travels. The man in black had passed through the town previously. After spending some time there, the leader of the local church becomes wary of Roland, and the town turns on him. In order to escape with his life, Roland is forced to kill every resident of the town, including his lover, Allie. Telling this story seems cathartic for Roland. When he is ready to leave the farm, his mule is dead, so he proceeds on foot. The farmer tells him that he is going to eat the mule.
At the way station Roland first encounters Jake Chambers, who died in his own universe when he was pushed in front of a car while walking to school. Roland is nearly dead when he makes it to the way station, and Jake brings him water and jerky while he is recovering. Jake does not know how long he has been at the way station, nor does he know exactly how he got there. He hid when the man in black passed by the way station. Roland hypnotizes him to determine the details of his death, but then makes him forget before he awakes. Before they leave the way station they encounter a speaking demon in the cellar while looking for food. After their palaver, Roland snatches the jawbone from the skeleton in the hole, from which the demon speaks.
After leaving the way station, Jake and Roland eventually make their way out of the desert into more welcoming lands. Roland rescues Jake from an encounter with an oracle, and then couples with the oracle himself in order to learn more about his fate and path to the Dark Tower. Roland gives Jake the jawbone from the way station to focus on while he is gone. After Roland returns, Jake discards the jawbone. As Jake and Roland make their way closer to the mountain, Jake begins to fear what will become of him.
During their travels to the mountain, some of Roland's childhood is developed, both through flashbacks and conversations with Jake. In a flashback, we learn about Roland and Cuthbert Allgood's chance encounter in a kitchen which leads to the hanging of Hax, the cook. The apprentice gunslingers are allowed to witness the hanging with their fathers' permission. Later, Jake asks Roland about his coming of age. Roland reveals how he was tricked into calling out his teacher Cort early, through the treachery of Marten. He succeeded in defeating Cort in battle through his ingenious weapon selection - his hawk, David.
Once they reach the mountain, and begin to travel through it, Jake's trepidation is even more clear. Even after the encounter with the slow mutants where Roland protects him, Jake does not fully trust Roland. When they are nearing the exit from the mountain, they must cross a fragile train trestle. When they are almost across, the man in black appears at the exit. Jake slips, and Roland must choose between saving Jake and following the man in black. Jake knows what his decision is: "Go then, there are other worlds than these." Jake falls to his death, and Roland chases the man in black.
Roland and the man in black hold palaver in a Golgotha. Using Tarot cards, the man in black tells Roland his future. Walter tells Roland that he will meet three persons in his quest: The Prisoner, a man possessed by a demon named "HEROIN", The Lady of the Shadows, a woman with dissociative identity disorder (i.e split personalities), and Death, but not for Roland as the Man in Black explains. After his encounter with the man in black, Roland sleeps for an impossible length of time, which is theorized by his Ka-tet later on to be centuries or millennia long. When he wakes, all that is left beside him is a pile of bones wrapped in the shreds of a black cloak and hood. Before proceeding further on his quest, Roland takes the jawbone of the skeleton to replace the one Jake had earlier discarded.
I really, highly recommend it.