The penultimate episode of "Orphan Black" Season 2 is titled "Things Which Have Never Yet Been Done." The episode sees the return of Helena and the Proletheans. The preview of the episode suggests that Sarah will find herself in a difficult position because of Cosima's heath.
In the previous episode, Cosima collapsed on the floor and suffered from a violent seizure. She vomited blood, as well. She is still alive in episode 9 but her condition is fast deteriorating. The preview clip reveals that it is Kira's bone-marrow that may hold the hope.
"The disease is spreading too rapidly. But, if we can re-boot her immune system ...," Delphine can be seen telling Sarah in the preview clip. Saran retorts before Delphine can complete her sentence. "With my kid's bone-marrow," Sarah can be seen saying.
At Dyad, things may get more complicated, as Marion will appear again. Michelle Forbes, who plays Marion, had said in an interview with The Hollywood Reporter that her character is there in the final two episodes of "Orphan Black" Season 2. Marion is set to go "weirder and weirder."
Meanwhile, Donnie and Alison may have some answering to do and the body of Dr Leekie to hide. The preview clip provides a glimpse of Vic asking where the garage is and also, Alison and Donnie can be seen throwing Dr Leekie's body in the chest freezer.
Donnie had accidently shot Dr Leekie dead, while Alison had let Aynsley choke to her death. During her rehab time, Alison had confessed to Vic about what she has done. It is possible that Vic comes knocking at the door of Alison and Donnie, with cops not far behind.
The preview clip also indicates that for the first time, Mark may speak out against Hank's decision. Gracie has been incarcerated and Mark can be seen saying, "You're not locking Gracie anymore."
It is a mere speculation at the moment but it might be possible that Helena's unborn may hold the key to Cosima's regaining her health.
Official Synopsis of "Orphan Black" Season 2 Episode 9, "Things Which Have Never Yet Been Done": "Sarah becomes frantic when Cosima's health take a down turn but her actions has heartbreaking costs for the others. The Proletheans implement their fundamental plan. Complications with the law involve Alison and Donnie."