I guess there are spoilers but I feel like everyone on earth pretty much knows what happens.
This was surprisingly unnerving, considering that I knew the essential story line, but perhaps that was what made it so suspenseful. I knew Dr. Jekyll and Mr. Hyde were one and the same, but none of the characters realize what's going on. The narrator, Mr. Utterson, hears from his cousin about a nasty man who runs over a small child; by the end of his cousin's story, Mr. Utterson realizes that the strange man is somehow entangled with his client and friend, the highly-respected Dr. Jekyll. Mr. Utterson is confused why an esteemed person such as Dr. Jekyll would associate with a disreputable character like Mr. Hyde and he assumes the poor doctor is being blackmailed over a youthful indiscretion. Poor Mr. Utterson is very wrong.
I haven't sorted out my thoughts yet on the duality of human nature, the good and evil co-existing within us all, the hypocrisy of Victorian morality, except that I found it tragic but utterly believable that once Mr. Hyde was unleashed, he soon took on a life of his own, thus condemning all that was good in poor Dr. Jekyll. I was a bit disappointed, however, that Dr. Jekyll was so vague about his depravity and evil doings. Obviously, RLS should have foreseen that modern readers crave the sordid details.