He is quickly set upon by several mutant lobsters ("lobstrosities") and loses two fingers on his right hand and a big toe. As he makes his way along the deserted and seemingly endless beach, growing weaker from infection, Roland will encounter three separate doors. Each door is a portal into "our" world - specifically, New York City at three different times - and it is through these doors that Roland will draw the people who will become his ka-tet*, his companions on his journey to the Dark Tower.
* According to the Dark Tower Glossary, Ka "signifies life force, consciousness, duty, and destiny" and tet is "a group of people with the same interests and goals".
Passing through the first door (on which is inscribed "The Prisoner") puts Roland in the mind of Eddie Dean, a heroin addict, who is at that moment (in 1987) on a flight into New York City, attempting to smuggle two pounds of cocaine for a drug lord named Enrico Balazar. At first, Eddie is more than a little freaked out to hear this other commanding voice in his head, but by the time the flight lands, he has adjusted, and Roland (who has tipped Eddie off that the flight crew is suspicious) has formulated a plan to get him safely through customs. Later, their bond is cemented when they survive an intense shootout with Balazar's goons. Eddie is also able to get some medicine for Roland's infection.
The second door ("The Lady of Shadows") brings Roland to 1964 and into the mind of Odetta Holmes/Detta Walker, a wealthy, disabled black woman involved in the civil rights movement. Odetta became schizophrenic after being struck on the head by a falling brick when she was five years old; Detta is her other personality, though each personality is not aware of the existence of the other.
That falling brick was not an accident. It was dropped on her by a sociopath named Jack Mort, who Roland encounters when he ventures through the third door ("The Pusher") and emerges in 1977. Mort is also responsible for pushing the adult Odetta onto the subway tracks where a train cut off her legs above the knees*. (Continuing the coincidences and connections, Mort also pushed Jake to his death in The Gunslinger, which causes Jake to be at the waystation at which Roland found him. (In this book, however, Roland prevents Mort from pushing Jake in front of the car; apparently, King will deal with this contradiction in the next volume.))
* Later in the book, King commits a big goof and has the A train come into the Christopher Street subway station. Christopher Street is used by the 1 train.
In the process of killing Jack Mort, Roland is also able to fuse the two personalities of Odetta into one woman*, who becomes Susannah Dean. (Susannah is Odetta's middle name. Eddie had fallen in love with Odetta while walking on the beach between the second and third doors, and although there is no marriage or other ceremony in the book, she has taken his name.)
* This process is hard to explain succinctly and seems, like some plot twists in the book, more than little contrived.
From The Complete Stephen King Universe:
[A]lthough they cross many miles of ground, that in itself does not feel like progress. The progress in this tale is almost entirely internal. ... [T]he only real progress is in the preparation to reach the Tower. The team is molded, the bonds are formed, almost as though these were necessary rituals.Once King determined he would return to the Gunslinger, and had a clearer idea of where he wanted to take his story, he needed to lay the foundation for the later volumes - establishing the backstory and identity of the main characters. That is what The Drawing of the Three does - and it gets a bit dull in places.
King's Afterword (written in December 1986) states that the Dark Tower series will contain six or seven books. He gives the titles for the next two: The Waste Lands and Wizard and Glass, which arrived in 1991 and 1997, respectively. King admits that he is both surprised and grateful at the reception the first volume received.
This work seems to be my own Tower, you know; these people haunt me, Roland most of all. Do I really know what that Tower is, and what awaits Roland there ...? Yes ... and no. All I know is that the tale has called to me again and again over a period of seventeen years.Next: Misery.