Spring comes and with it a mysterious event that introduces Jim to a new facet of the Captain's personality. Jim's father's health worsens with time, so Jim and his mother take charge of the Inn. On a frosty January morning, the Captain steps out of the Inn with his usual accessories quietly, as though the thought of Dr. Livesey still weighs in his mind. As Jim is setting the breakfast table, in comes a man wearing a cutlass, whom he had never seen before.
While he orders rum, he tries to get friendly with Jim and inquires if the breakfast in the table is spread out for his mate Bill'. On hearing that it is for the Captain, he asks if his Captain has a cut on his right cheek. This matches the scar Jim observed when he first saw the Captain. Assured that the Captain is his mate Bill, he asks other questions and waits for him out side the Inn. Jim finds something weird in the stranger's behavior when he is asked not to go outside. His confusion doubles when the stranger wants to surprise the Captain by hiding behind the door.
When the Captain returns, Jim finds him uneasy and disturbed, rather than surprised to hear his name being called. Recognizing his mate Black Dog' he quickly gets to the point without any small talk. Jim is told to leave them alone after serving them rum. Jim's eavesdropping doesn't give him any clue about the two sea mates, for he can hear only slow indecipherable sounds. When the sound changes to thuds and clinging of steels, Jim witnesses a battle raging between Billy and the Black Dog. Black Dog manages to escape despite having a cut from the battle on his right shoulder.
Asking for another drink, the Captain then collapses on the floor. Jim and his mother feel helpless. In an effort to help the Captain, Jim tries to pour some rum down his throat. Dr. Livesey, who is on another routine visit to check Jim's father, takes a look at the Captain. After examining him, he tells them that the Captain has had a stroke. As a precautionary measure, Jim's mother is asked not to tell her husband what happened, so as not to upset him. Dr. Livesey, to save the Captain, cuts open a vein from his arm and in the process exposes some of the tattoos on the Captain's arm which are comprised of various writings and a well sketched tattoo of a man hanging from the gallows.
After forcing a great deal of blood from the Captain's body, he regains his consciousness. The doctor tells him what has happened and asks him to stop drinking if he wants to live. But the Captain is more concerned about Black Dog. He is quite irritated when the doctor addresses him as Mr. Bones.
Jim and the doctor help him to his room and after putting him in bed, the doctor in his authoritative tone warns the Captain continuing to drink will kill him. On his way to check Jim's father, the doctor tells him that he has drawn enough blood to keep him quiet and that he should rest for a week. Finally, he also tells Jim that if the Captain continues to drink, his death is certain.
This chapter builds on the mystery and adds an extension to the personality of the Captain. In this chapter we learn the Captain's first name is Bill. Another character is introduced - Black Dog. He tries to get friendly with Jim, but Jim finds him odd when he asks Jim not to step out of the inn on the pretext of giving surprising his friend Bill. The Captain, instead of being surprised, reacts strangely when his name is called. Recognizing him instantly, the Captain asks him the purpose of his visit. Black Dog and Bill sits down to settle scores and Jim is asked to leave. Curiosity, one of Jim's predominant qualities is emphasized by Stevenson. Wanting to know more and more about them prompts him to eavesdrop. Jim hears only muffled sounds but he knows that it is not a friendly conversation, then leading to a fight. This incident highlights the fact that children have a natural inclination for curiosity and that they are also natural eavesdroppers.
Jim's relief when the doctor saves the Captain shows his sympathetic nature. Jim is taken into confidence by the doctor and asked not to provide alcohol to the Captain.
Jim is portrayed as a caring, obedient, and responsible boy by Stevenson. He also tells us a little more about the Captain's personality. But, we don't know much about the strange 'Black Dog'.