The BBC America series Orphan Black is about...well, it's difficult to say what it's about without giving away spoilers. The first episode opens with a young, streetwise grifter named Sarah arriving at a train station, where she witnesses another woman folding her clothes carefully on the platform and taking off her shoes. The woman turns, looks at Sarah...and Sarah sees her own face looking back at her. Then the identical woman walks in front of an oncoming train to her death. Sarah steals the woman's purse and clothes, gets her name and address from her ID, and decides to sneak into this woman's life to find out why they look the same. From there, things just get stranger and stranger.
Orphan Black is one of those very rare TV shows: true science fiction, working off of the extrapolation of real science, addressing serious scientific and social questions, and paced like a novel. Each episode is fast and tightly paced, but the series itself unfolds slowly, doling out information and answering questions conservatively. The real premise of the show isn't revealed until the third episode, and the title of the series isn't really made explicit until later in the first season. The characters and relationships are all complex and layered. Everyone has strengths, weaknesses and secrets, and the way people relate to each other is constantly shifting.
The acting in Orphan Black is excellent, but the real breakout is star Tatiana Maslany. She plays up to eight different characters, and she plays them all so differently--with different accents, mannerisms, body language--that you'd swear it's eight different actresses. She's absolutely amazing to watch.
The second season is currently airing on BBC America, but if you haven't seen it yet, check out season one and prepare to have your mind blown.