I'm sure I'll get in trouble with the Steven Moffat fans for comparing him to Joss Whedon, but I cut my teeth on Whedon-style, so that's the comparison I'll make.
Jekyll was a six-episode BBC series from 2007. It was written by Doctor Who scribe, Steven Moffat, and starred James Nesbitt, an actor from Northern Ireland known in the UK for roles in Cold Feet and Murphy's Law. Nesbitt played Dr. Jackman and his mysterious alterego, Hyde. The show also featured Michelle Ryan, as a psychiatric nurse hired by Jackman & Hyde to assist them, and Gina Bellman as Claire Jackman, the doctor's wife.
I caught wind of the show while doing someo f my more eccentric online research on werewolves; the Jekyll and Hyde story is often considered a variation on the werewolf theme. I was intrigued enough to track down a copy of the DVD set at the library and started watching. Nesbitt is amazing as the title characters. He wears very light prosthetics and manages the character change mostly through his acting and the acting of the supporting cast.
The plot is a sequel or continuation of the original story. Dr. Jackman begins to experience episodes in which the Hyde persona emerges. He realizes this is not a split personality, as the other persona actually exhibits different physical traits. He then pursues the mystery and discovers a secret history and conspiracy afoot.
The writing is clever and in turns, dramatic, comedic, and scary. Hence, I compare it to Joss Whedon's best work on Buffy and Angel. If you're into those shows, this is worth tracking down; you won't be disappointed.
Not to mention that Ryan and Bellman are HAWT! Did I just print that? Oh, well, too true not to point out.