Henry Jekyll is a well respected member of London society. In his professional life, he is a medical doctor, scientist and academician.
Dr. Henry Jekyll believes that there are two distinct sides to men - a good and an evil side. He believes that by separating the two man can become liberated. In his experiments, he uses himself as the subject to test his hypothesis. His evil side, who he coins Mr. Hyde, escapes into London, and terrorizes the city. When Hyde drinks the potion, he transforms back into Dr. Jekyll.
When Henry is Hyde, he is purity evil and free to be wicket without hurting Dr. Jekyll’s reputation. When Henry is Dr. Jekyll again, he is not purely good, but a mix of good and evil.
"Hence, although I had now two characters as well as two appearances, one was wholly evil, and the other was still old Henry Jekyll, that incongruous compound of whose reformation and improvement I had all ready learned to despair. The movement was thus wholly toward the worse."
As time goes by, it becomes easier to transform into Hyde, but harder and more painful to transform back into Jekyll. Soon, Henry starts transforming into Hyde without the aid of the potion.
Henry realizes that he can’t be both men, but he will have to choice one twin and stay him. By then, it is too late. Hyde has taken over, and Henry can’t transform back into Jekyll, no matter how much of the potion he drinks. He is trapped in Hyde’s body. He chooses suicide rather than enslavement to his monster.