'Orphan Black' Season 2, episode 7 'Knowledge of Causes, and Secret Motion of Things' recap
Sarah, Alison and Felix managed to team up to neutralize the Deangelis threat, as they realized Vic was going to give up details on Alison in exchange for his charges being dropped, while Sarah and Alison were forced to trade places during family day at rehab, resulting in Sarah pretending to be Alison pretending to be Donnie, a.k.a. just another Tuesday for Tatiana Maslany and her otherworldly acting talent.
Donnie also found out the truth about Sarah -- he had no idea about the whole clone thing; he'd just signed up for some sociology experiment. He confessed as much to Alison, who informed him that he's the one who'd ruined their life together.
Hurt, Donnie managed to track down Leekie just as the Doctor was escaping the Dyad (and the wrath of Rachel and scary boss lady now that Rachel's dad was back in the picture). Rachel let him go out of sentimentality, but Donnie accidentally finished her job and shot Leekie in the head, splattering his brains all over his car. Oops!
Meanwhile, Mrs. S shepherded Duncan out of his hoarder house, and found out he had floppy discs (like, the actual literal floppy kind from the '80s) full of information about the whole project.
Cosima discovered that her treatment was actually stem cells from Kira, but once Sarah found out it was working she volunteered another of Kira's lost teeth to her sister of sorts.
Also, Cal is gone. Is he gone gone, though? Kira and Sarah are reunited, but hopefully that doesn't mean his beautiful face is out of the picture for good.