[Warning: This post contains spoilers from Episode 6 of Star Trek: Discovery, "Lethe." Read at your own risk.]

Sunday night's Star Trek: Discovery ventured deep into the mind of Ambassador Sarek (James Frain), divulging a violent internal conflict brewing underneath that steely demeanor.

In the emotionally-charged episode, an assassination attempt leaves Sarek gravely wounded and Burham (Sonequa Martin-Green) — sharing a part of his Katra — senses his impending doom and convinces Captain Lorca (Jason Isaacs) to send the Discovery to his rescue. While en route, the delirious Vulcan accidentally pulls her into his conscience, revealing a bold choice that has left him with great shame.

As he lay dying, his mind goes to the memory of the day that Burnham was rejected from the Vulcan Expeditionary Group. Faced with the reality that only one of his children would be allowed admission, he was forced to choose between Burnham and Spock, and he chose the latter. It's a decision that still weighs heavily on him, especially since he lied to Burham and made her believe she had failed him.

"I think that he understands at a certain point that she needs to be with humans and that in some way, he's put her in an impossible position," Frain tells TV Guide of Sarek's choice. "His son is half-Vulcan and she's all human so it's like who's got the most Vulcan in them? It's a horrific choice but the logical choice is to say that he has to choose Spock. He can't set his son up for failure. "

With the truth out in the open and Burnham mind-melding with him to save his life just as he did with her, the two share a special connection. Despite that intense emotional journey together, Sarek doesn't automatically open up. Instead, he pretends not to remember what happened during the mind-meld, again putting up an invisible wall between them. While it might seem like a callous move, Frain insists it's more of a means of protection for Sarek.

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"She manages to get out of him something so painful and vulnerable and something he's not necessarily equipped to handle. At that moment, he's not ready," he explained. "It's not cruel, necessarily."

Sarek's act of self-preservation certainly doesn't move their relationship forward, but Burnham is convinced that one day he will finally open up to her. Frain, too, admits that that might be possible. "The seeds have been sown. It's undeniable what's occurred," he said.

To get to that place where he can fully connect to her, however, he'll need to push past his Vulcan nature which rejects such a human display of emotion."He's brought up in this very very strict, almost kind of samurai code. He can't step out of that. And so his idea of what is right is always being challenged but he's certainly always trying to do the right thing," added Frain.

The episode also saw newcomer Lt. Ash Tyler (Shazad Latif) officially join the USS Discovery as its new chief security officer. We first met him in last week's episode,"Choose your Pain," as Lorca's cellmate aboard a Klingon prison vessel. The two eventually bust their way out, with Tyler gaining much of Lorca's trust in the process. "They've definitely bonded. They've spent time together and they've gone through this harrowing escape experience. There's a bond there and there's a trust there," Shazad Latif tells TV Guide.

Though he escaped from the Klingons after spending seven months as their captive, it's obvious that there are deep emotional scars — as seen in his violent takedown of L'Rell in that hallway during their escape — that he'll need to contend with at some point. "He beats her to a pulp and then he leaves. That's gonna be stained on his mind. She's definitely left a mark on him."

Yep, there's some unfinished business indeed.

New episodes of Star Trek: Discovery stream Sundays at 8:30/7:30c exclusively on CBS All Access.

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James Frain, Star Trek: DiscoveryJames Frain, Star Trek: Discovery