Now that’s how you do a surprising death.

In this episode, Dr Leekie goes from being potentially Sarah’s greatest enemy, to threatened by Rachel after she learns the truth, and then to… murdered, accidentally, by Donnie. You don’t get much more unpredictable than that.

Last week it seemed natural to theorise that Rachel’s reaction to discovering the truth of her mother’s death would completely change things within the Dyad Institute. It does, but the causation is much less straightforward than that. Dr Leekie raised Rachel, and she does care for him.

We expect her to coldheartedly kill him off after she meets with her dad and is told who really caused the fire, but what happens is more pragmatic and shows her respect for him. As she says, “It’s foolish to spare you, but you raised me. Nurture prevails.”

Instantly, the suggestion that Leekie is Orphan Black’s main villain is deflated. He becomes a whimpering mess in front of Rachel, hardly the evil mastermind you’d expect. Our new threat to the clones is much more similar to Rachel in her outlook. Yes, Rachel’s boss Marian Bowles (Michelle Forbes) is ruthless in her approach, declaring Sarah a distraction and effectively organising the execution of Leekie. Rachel doesn’t kill him, of course, but he still ends up dead after a confrontation with Donnie goes completely wrong.

There’s no sign of Helena this week, but we do get a tremendous series of events at rehab with Sarah attempting to appease Vic, then having to impersonate Alison while Felix and Alison try to remain hidden from Angie.

Watching clones impersonate each other is always great, and it’s even better when you realise Donnie has absolutely no idea what’s going on. When he stumbles into Alison’s room to see the pair of clones, he’s dumbfounded. This revelation, coupled with Alison telling him that he’s ruined their family, is what makes him go after Leekie.

That drama forms the spine of ‘Knowledge of Causes, and Secret Motion of Things’, but the science doesn’t go unmentioned. Delphine’s secret doesn’t remain secret long, as Cosima learns that it’s Kira’s cells she’s being treated with. This season, the clones have been keeping secrets from each other far less, so it’s unsurprising to see Cosima tell Sarah what’s going on.

What is surprising is how quickly Kira’s ripping out a tooth to help. It’s almost like she knows what’s going on ahead of time. This feels like it may be a pause in Cal’s arc, though, as Sarah resolves to help Cosima but not to tell Cal much at all about what she’s doing.

As always, trust is at the heart of Orphan Black. The show seems cautious about telling us that being honest is particularly useful, though.

Every instance where the truth is revealed, the consequences can’t be predicted by the characters: Donnie is far from rational, Rachel isn’t as merciless as she could be, Vic isn’t who Alison thought she was, Sarah isn’t who Vic thought she was, and Delphine’s lies may have destroyed her relationship with Cosima. The one thing that we can count on is, as Paul says, that Mrs S and Sarah have “a knack for burning things down”.

Aired at 10pm on Wednesday 11 June 2014 on BBC Three.