Hiya! I read this book a couple years back. Hope I'm not too rusty.
Dr. Jekyll is a scientist. He made a potion that, when he drinks it, the evil comes out of him as a new person, Mr. Hyde. Dr. Jekyll was immoral when he was younger, causing trouble and enjoying himself. Once he grew older and wiser, he was starting to wish he was young again, and careless. He was always dabbling in the "Dark Arts" or the unknown places. But he eventually morphed into Mr. Hyde after making a potion that would take the evil out of him and make two people, Mr. Hyde and Dr. Jekyll.
Dr. Jekyll is intended to be the nice one. He is a well-respected scientist.
Mr. Hyde is a part of Dr. Jekyll, that Dr. Jekyll created. Dr. Jekyll liked going into Mr. Hyde and playing as him so that he could enjoy himself and be careless without being caught.
So Dr. Jekyll's whole body was turned into Mr. Hyde one night, and he couldn't get himself out of that form. Dr. Jekyll was afraid that his friends, Mr. Utterson, Mr. Enfield and Poole, his butler, would find him as Mr. Hyde. Everyone knew that Mr. Hyde was not nice, being that Mr. Hyde ran over a child with his carriage, killed a doctor by beating him to death, and more. Dr. Jekyll had left all in his will to Mr. Hyde, which they found suspicious.
So eventually Mr. Utterson (Dr. Jekyll's lawyer), Mr. Enfield (Mr. Utterson's cousin), and Poole (Dr. Jekyll's butler) broke down Dr. Jekyll's door because they were worried about him, and found that Mr. Hyde had committed s*icide in the house. Mr. Utterson eventually found the letter addressed to him from Dr. Jekyll and found out that Mr. Hyde was Dr. Jekyll.
There ya go!