The past four seasons of Orphan Black have been all about digging. From the topsoil mystery of Sarah Manning's doppelganger throwing herself in front of a train, down through the layers of Neolution and Dyad and BrightBorn, to where the show's season 4 finale ends up: with Cosima (Tatiana Maslany) and younger clone Charlotte (Cynthia Galant) landing at a secret camp hidden away on the "Island of Dr. Moreau" - where the true father of Neolution, P.T. Westmoreland (Geza Kovacs), is still alive despite his impossibly advanced age. And with just one more season to go, we might be close to hitting rock bottom.

Ironic, really, because Orphan Black has never been better. "From Dancing Mice to Psychopaths" contains as much intrigue and mystery as the early episodes of season 1; just when the show seemed to have laid all its cards on the table, it pulls another from out of its sleeve. For some viewers the constant reveals that the person supposedly at the top of the food chain is just working for a higher power might seem a bit exasperating at this point, but it definitely seems like the core of the conspiracy has finally been exposed.

The finale is bookended by the return of Delphine (Evelyne Brochu), once believed dead but now revealed to have been hiding out in Westmoreland's camp, on the same island where Cosima has been conducting her stem cell research alongside Susan Duncan (Rosemary Dunsmore). A romantic might say that destiny is drawing Cosima and Delphine together, but in Orphan Black it's more likely the result of behind-the-scenes manipulation. The seasons-long search for a cure for the clones' sickness finally bears fruit with the successful fertilization of one of Sarah's eggs with Ira's (Ari Millen) sperm... just in time for Susan to betray the clones and take the research for herself, with plans to restart her human cloning program.

The line between hero and villain has always been a little blurry on Orphan Black - and not just because Alison and Donnie (Kristian Bruun) can't seem to stop killing people. Easily one of the most interesting character arcs this season was that of Rachel, who has been slowly recuperating ever since Sarah stabbed her in the eye with a pencil at the end of season 2. With Rachel and Sarah at one point forced into an uneasy alliance, it seemed like there might be hope for the former head of Dyad to become a member of the Clone Club - who knows, perhaps even calling her fellow clones "sestra" one day.

But Orphan Black is all about agendas; everybody has one, and Rachel's fellow clones do not factor into hers. After spending this season in particular seeing Rachel in her most vulnerable and sympathetic moments, it's thrilling to see her claws come out again as she double-crosses Evie Cho (Jessalyn Wanlim), Susan Duncan, Ira, and her fellow clones in the space of just two episodes - all in the interest of getting back into a position of power among the Neolutionists. All of the clones are formidable in their own way (it must be in the genes), but Rachel is the only one with a real lust for power, and it's been a lot of fun watching her manipulate, blackmail and murder her way to the top.

Of course, not all of the Leda sisters are genius scientists or cutthroat tacticians. Playing an important role in this episode is sort-of-self-aware clone Krystal Goderitch - a character who was originally supposed to be killed off in the same episode that she was introduced, but who won a stay of execution from showrunners Graeme Manson and John Fawcett because they loved Maslany's take on the character so much. Krystal largely serves as a comic relief character (she thinks that Estée Lauder is on the conspiracy - and given Orphan Black's twists and turns, she could be right), but she does lead Sarah and co. to the information that BrightBorn's Dr. Van Lier (Scott Wentworth) was responsible for saving Delphine's life and spiriting her away to the island.

The finale comes to a satisfyingly tense climax on the island, as Rachel, Susan and Sarah all converge in a struggle that ends up spilling a lot of blood. Rachel stabs Susan in the stomach and Sarah in the thigh, and ends up climbing dominion of the island's manor (with Susan seemingly left to die in Rachel's old wheelchair). Sarah escapes, wounded, but in her absence Siobhan (Maria Doyle Kennedy) and Kira (Skyler Wexler) have fallen into the hands of Rachel's ally (and kinky bed partner) Ferdinand (James Frain).

Following a slightly weaker third season, Orphan Black season 4 has definitely pulled things together in time to set the stage for what will hopefully be an equally excellent final chapter, with Rachel reestablished as a powerful antagonist - potentially alongside the mysterious P.T. Westmoreland. Will the Clone Club ever escape the clutches of the science that created them? And how will Neolution's plans come to shape the future of humankind? After this finale, fans will definitely be eager to find out.

Orphan Black will return to BBC America for its fifth and final season 2017.