What constitutes a family? Are they the people who give you life, the people who raise you, or the people you choose as your support network? Or are they your identical clones created by a mysterious organization seeking to advance human evolution to the next level? That last one might not be a question most family dramas are interested in asking, but Orphan Black isn't most family dramas.

Like the best sci-fi shows, BBC America's addictive Orphan Black uses its fantastical lens to explore realities of the human condition. Where Battlestar Galactica examined politics and terrorism using a fleet of spaceships and Buffy the Vampire Slayer depicted the struggles of adolescence through demons and witches, Orphan Black uses human cloning to explore the nature of family. That unifying central theme, a slew of fantastic characters, and an absolutely stellar central performance (well, performances) from star Tatiana Maslany combine to make Orphan Black one of the best shows of 2013 and one you should absolutely check out before it returns for a second season on April 19.

The heart and soul of Orphan Black is Canadian actress Tatiana Maslany, and it would be virtually impossible to oversell her talents. Over the course of the first season she plays four leading roles and three featured ones, turning each into a recognizable human being with her own physicality, vocal quality, and mannerisms. She occasionally plays characters pretending to be other characters as well, and what sounds confusing on paper (Helena poses as Beth, while Sarah-as-Beth is away from the office) becomes clear storytelling in Maslany's capable hands. Refreshingly, Orphan Black doesn't use its one-actor-plays-multiple-roles conceit as a cheap gimmick; it simply calls upon its lead actress to inhabit the full reality of each of her characters. Maslany's performance deserves ample praise—and every award they can throw at her—but Orphan Black is much more than just one impressive performance.

And that's because the show's ever-complicating plot is grounded by solid character work all around. Well before the sci-fi elements are introduced, the show raises questions about the nature of family through the rather unconventional one at its center. The first episode, "Natural Selection," opens with Sarah Manning—a British punk rocker and con-woman—returning to town (vaguely Ontario) to reclaim her seven-year-old daughter, Kira. Sarah's biggest ally is her foster brother Felix—a gay rent boy and artist who's alternately the show's comic relief, its moral compass, and its secret weapon (actor Jordan Gavaris is the only one who comes anywhere near Maslany's level). They may not be biologically related, but Sarah and Felix's sibling bond is probably the most grounded relationship on Orphan Black. Which is good because Sarah desperately needs all the help she can get to win back the daughter she dropped off for a one-night stay 10 months ago. Felix and Sarah's former foster mother Mrs. S (Maria Doyle Kennedy) has been taking care of Kira ever since, and after years of dealing with Sarah's reckless behavior, she isn't too keen to relinquish custody.

Thankfully, Sarah's uncanny ability to get herself into and then work her way out of tight corners is one of Orphan Black's most engaging qualities. After seeing a woman who looks just like her commit suicide-by-train, it takes Sarah all of eight seconds to decide to steal the dead woman's purse, break into her apartment, and form a plan to impersonate her and steal the $75,000 she has in a savings account. With that money Sarah will finally have the financial stability she needs to start a new life with Kira and Felix. The only hitch is that the life of Beth Childs—Sarah's doppelganger—is not nearly as perfect as her stunning abode and equally stunning boyfriend would suggest.

The first three episodes mine most of their drama from watching Sarah-as-Beth squirm to manage a disciplinary hearing, a tense relationship with Beth's co-detective Art (Kevin Hanchard), and the suspicions of Beth's hunky-but-boring boyfriend Paul (Dylan Bruce). Where a lesser show might introduce a deus ex machina to get Sarah out of these tight spots, Orphan Black lets its protagonist be incredibly crafty—confidently lying, flirting, and faking her way out of danger. She's always just one step ahead of getting in too deep, until the third episode "Variation Under Nature," when she finally uncovers Beth's biggest secret: She was one of many identical clones living all around the world, and before her death she was working with two of them to investigate their past.

It's admirable that a show about clones takes three whole episodes to even introduce the word, an example of Orphan Black's supreme confidence in its world building. Once the concept is introduced, however, it raises new questions about the nature of family. Sarah's biological daughter and her adoptive brother are clearly her family, but what relationship does she have to these other women who look just like her? Biologically identical but raised in separate environments, what—if anything—do these girls owe to one another? The mystery of their creation ties them together into a "Clone Club," but is their bond professional or personal? After all, Sarah could not be more different from her clone "sisters" (let's call them "clone-sters" for short).

Alison Hendrix is a type-A soccer mom with a nebbishy husband named Donnie, a fabulous color-coordinated craft room, and two adopted kids (Sarah appears to be the only clone who was able to conceive a child). Cosima Neihaus is a dreadlocked, tattooed grad student studying evolutionary development (that's "evo-devo"), with a soft spot for gorgeous French women. With Cosima offering the brains and Alison bankrolling their operations (with that $75K), Beth rounded out the Clone Club with her pragmatism and access to police intel. With Beth dead, Sarah fills the much-needed role of group badass. United by the mystery of their creation and the threat of death—there were at least four European clones who were either assassinated or killed by a potentially-hereditary disease—the Clone Club's investigation into their past keeps Orphan Black's momentum moving forward at a breakneck pace and also serves as the backdrop for some of the show's most compelling inter-clone relationships. Perpetual-student Cosima easily slots into the role of "younger sister" of the group while Alison and Sarah bond over their mutual motherhood. (It's easy to forget that the same actress plays all three women, as Maslany's unshowy trio of performances keeps the focus rightfully on the relationships.)

The show smartly extends its thematic exploration of family to its antagonistic forces as well, giving some much-needed humanity to the shady organizations that seem requisite to the sci-fi genre. At the end of episode three, Sarah-as-Beth discovers the person who has been assassinating the clones is in fact another clone—a blonde, feral Ukrainian named Helena. Raised by a malevolent religious order known as the Prolethians, Helena was brainwashed by her abusive father figure named Tomas and trained to rid the world of unnatural clones. Through Helena (Maslany's entirely unhinged, often hilarious iteration), the show explores the dark side of family. Instead of offering love and encouragement, those central emotional bonds can also be used for more sinister purposes.

While Orphan Black has yet to fully explore the nature vs. nurture debate it seems tailor-made for, it comes closest in its juxtaposition of Helena and Sarah. The penultimate episode, "Unconscious Selection," reveals that Helena and Sarah are actually sisters—a single egg that split into twins in their surrogate mother's womb. Despite being raised thousands of miles apart, Helena and Sarah have a bond they don't share with their fellow clone-sters. Orphan Black at first seems to be making a case for the power of biological sisterhood over artificially created clone-sterhood, but the season finale takes a sharp turn in the opposite direction. When push comes to shove Sarah kills Helena in order to protect the family she chooses—cloned, adopted, and otherwise—over the one she shared a womb with.

As with the nature vs. nurture debate, Orphan Black debates religion vs. science in only the loosest sense. On the flip side of the religiously fanatic Prolethians are the scientifically fanatic Neolutionists, a mysterious institution attempting to artificially speed up human evolution by giving themselves tails and running nightclubs. The Neolutionists' Dyad Institution created the clones back in the 1980s and now that they've become self-aware, Dr. Aldous Leekie (Matt Frewer) wants to work in partnership with his "daughters."

Yet the Neolutionists are not the benevolent parental figures Leekie makes them out to be. It's one of the show's creepier reveals that the clones are being unwittingly monitored in their sleep every so often. The aforementioned Paul (he was Beth's monitor in addition to her boyfriend) and Cosima's monitor Delphine (Évelyne Brochu) struggle between their professional duties as observers and their emerging affection for their subjects. These tortured relationships are meant to give the show a dose of sexual tension, but Orphan Black is much better at creating familial relationships than romantic ones. Cosima and Delphine work just fine, but Paul and Sarah's romance is too often a drag on the first season's pacing.

But while the show hasn't quite figured out what it wants to do with Paul (and actor Dylan Bruce has little to bring to the table), that's a minor misstep given how well Orphan Black manages to flesh out its other characters in the span of only 10 episodes. Creators Graeme Manson and John Fawcett have always maintained Orphan Black is a character drama first and a sci-fi adventure second, and the show has a knack for pairing off its characters in unlikely ways and then slowly figuring out just what they have in common. While it would have been unthinkable in the first few episodes, by the end of the season it makes perfect sense that Felix is giving Alison a brotherly pep-talk or that Cosima shows up on Sarah's door in need of a hug.

After spending a season carefully building bonds between its characters—transitioning them from strangers to friends to family—the show proceeds to blow most of those relationships up in the explosive season finale, "Endless Forms Most Beautiful."

Alison—looking to take her life into her own hands—decides her nosey neighbor Aynsley is her monitor and allows her choke to death as revenge (in a scene that doubles as a PSA about wearing scarves near garbage disposals). Wracked with guilt, she lets Donnie's idea of "starting over" serve as her clean slate. She signs a contract agreeing to work in conjunction with the Neolutionists, just as the audience receives the gobstopping reveal that Donnie has been her monitor all along (sorry Aynsley). Cosima, meanwhile, appears to be sick with the same respiratory disease that infected some of the European clones, Paul has yet to decide if he's fighting for the clones or against them, and Sarah finds evidence that Mrs. S has been working with the Neolutionists all along.

Helena put a familiar face on the Prolethians and the finale reveals there's an identical one behind the Neolutionists as well. It turns out Leekie is just a flunky working for the Neolutionists' true leader, Rachel Duncan, a "pro-clone" raised by the organization (Maslany's chance to play an icy, bobbed Brit). Rachel's relationship with her clone-sters in the second season will be made more difficult by the fact that Cosima and Sarah now know the Dyad Institute built a patent right into the clones' DNA—making them property rather than people.

The episode's final reveal—that the Neolutionists have kidnapped Kira—leaves Sarah helplessly screaming out a window for her daughter. (It's appropriate that the season begins and ends with Sarah longing for Kira from afar). Before learning about her clone heritage, Sarah survived by keeping a narrow focus on the two people she cares most about: Felix and Kira. Season one complicated her life with a myriad of new family-like relationships, but without her laser focus, Sarah slipped up and put her daughter in danger. With Kira missing, Sarah's allegiance to her new found clone-sters will be put to the test. Sarah Manning is at her best when she's fighting for her family; season two will have to figure out exactly who they are.