The final act of “Scarred by Many Past Frustrations,” which, as it happens, is an appropriate description for viewers who’ve struggled to connect with Orphan Black this season, is nothing less than a salve. After two weeks in the weeds, “Newer Elements of Our Defense” offered, as I wrote, glimmers of hope for a return to form, and the conclusion of tonight’s episode makes good on the promise in spades. Beginning with the soused, lighthearted dance party among Felix (Jordan Gavaris), Mrs. S (Maria Doyle Kennedy), and the scantily clad Gracie Johanssen (Zoe De Grand’Maison), “Scarred by Many Past Frustrations” speeds through tonal registers and narrative twists, devoting particular attention to the relationship we’ve come to care about most, that between Helena and Sarah (Tatiana Maslany). Reaching into the characters’ pasts and pressing forward with ruthless betrayals and shocking revelations, the episode may turn out to be a one-off, but it roped me back in with a reminder of what the series is capable of. “Scarred by Many Past Frustrations” is vintage Orphan Black.