Count Vardulon turned me on to this one. Previous to this, my only encounter with the story of Dr. Jekyll and Mr. Hyde was that Loony Tunes cartoons where Tweety drinks the potion and turns into this massive thing that terrorizes Sylvester.
Dr. Tom Jackman is estranged from his wife and has quit his job. He lives in a flat in London with a psychiatric nurse and what looks like an electric chair. The nurse monitors his mental health, while the chair is used to contain Dr. Jackman each night when he turns into Mr. Hyde. Tom and Hyde have worked out a deal in order to share the body, but as Hyde grows stronger he becomes more willful and harder to control. To make matters worse, Hyde finds out about Tom's family while Tom finds out he's being followed by a shady and powerful organization who want to get their hands on Hyde.
The series is six episodes long, brilliantly and entirely written by Steven Moffat. Interestingly, Hyde is not the antithesis of Tom, he's not his opposite. Hyde is a child, which makes him impulsive and amoral. But he's also strong, fast, and hyper aware, which makes him dangerous. Tom does what he can to protect people from Hyde, and the two have worked out a deal to share the body so that neither one has to suffer much from the exploits of the other. But Tom doesn't fully understand why this is happening to him and though he is told time and again that Robert Louis Stevenson's wrote about a real man that lived Victorian London, he has a hard time believing he is the direct descendant of Dr. Jekyll. As the story progresses, Tom comes to learn more about the dark truth of his condition while Hyde, for his part, becomes increasingly and grudgingly involved in protecting Tom and his family from harm.
The titular character is played by James Nesbitt and it is his transformation from one personality to the other that really makes the show. What's even more amazing is the transformations themselves are never actually shown--they happen in the dark. There are few physical differences between Tom and Hyde, the main one being that Hyde is a bit thinner, so the real strength of the portrayal lies on Nesbitt's ability to play both roles convincingly.
Clocking in at just under six hours of television, Jekyll is not a huge time commitment, and it is worth every. Single. Minute.