After last week’s “Vaulting Ambition,” which felt like three interwoven one-act plays, this week’s “What’s Past is Prologue” is a fast-paced, single-story episode that gets right to the heart of Star Trek: Discovery’s Mirror Universe arc. Why is Discovery in the Mirror Universe, and how is it going to get back home?

“What’s Past is Prologue” opens with Lorca rescuing his compatriots from agony booths and continuing his march toward the emperor’s throne. Throughout the episode, the soundtrack to Lorca’s coup is his own voice. Star Trek has never shied away from making political statements, and Discovery’s most overt (so far) come in the form of Lorca’s ship-wide monologues.

Included are allusions to a few US presidential campaign slogans, as well as more generic but no less identifiable remarks proclaiming the supposedly objective superiority of one group over all others.

The source of Lorca’s “Make the Empire glorious again” is obvious; his overture to Burnham that she “help [him] bring peace to this world, through strength” is perhaps less so. For those who may be too young to remember, Reagan campaigned for the presidency in 1980 with the slogan “Peace through Strength.”

Jason Isaacs, whose acting has become especially impressive over the last few episodes, is clearly enjoying himself playing Lorca as the somewhat unhinged monster that’s apparently always been lurking under the surface. Isaacs doesn’t get hammy or chew scenery, but there’s a new and unnerving sparkle in his eyes when he looks at Burnham.

Lorca doesn’t present himself as a moustache-twirling villain, but as a charismatic leader. To a certain extent it’s understandable why people would want to follow him – both in the Mirror Universe and the Prime – and this makes his rhetoric even more dangerous.

In “What’s Past is Prologue,” we meet the Mirror version of Ellen Landry, Lorca’s former security chief aboard Discovery who was killed by “Ripper” the tardigrade back in “The Butcher’s Knife Cares Not for the Lamb’s Cry.” That Mirror Landry seems essentially identical to her Prime counterpart is a good indicator of just how easy it is to be swept up by Lorca’s magnetism.

In fact, Prime Landry was the first Discovery character who had me wondering if the Mirror Universe had somehow infiltrated the Prime. Watching Prime Landry coldly advocate for the torture of “Ripper” back in Episode 103, it’s not hard wonder if all Ellen Landrys — in all universes — are such sadists. Or perhaps Lorca is charismatic enough as a leader to make a Starfleet officer indistinguishable from her Mirror self. And isn’t this the more frightening proposition?

“What’s Past is Prologue” is, first and foremost, an episode about leadership. Lorca leads from a place of fear, and the speeches he makes while overthrowing Georgiou underline the Terran Empire’s totalitarian worldview. In Saru we find a leader who acknowledges his fear and steps beyond it to do what’s right.

Saru’s transformation from lieutenant to captain has been slow and steady, as befits a Starfleet officer who doesn’t have the luxury of killing his way to the captain’s chair, but ultimately so much more satisfying to witness than Lorca’s coup. It’s especially poignant that, given his relationship with fear, Saru finds a calm, confident command presence in what is likely the most terrifying place he’s ever been.

Even with all the speechifying, “What’s Past is Prologue” is an action-heavy episode, and thanks to beautiful lighting and direction and excellent choreography, that action never feels repetitive. The corridor phaser fight feels tactical in a way that many older Star Trek phaser battles simply don’t.

I will say though, I wish the sound effects had been toned down a bit. All those phasers powering up and down made for a lot of sound effects in a short period of time, to the point that it bordered on parody and took me out of the moment.

Meanwhile aboard Discovery, Lt. Stamets and Cadet Tilly come up with a plan to destroy ISS Charon’s energy source and ride the shockwave back to the Prime Universe. The glowing sphere I thought was a singularity is actually a mycelial reactor, and its continued operation will kill the entire mycelial network, and soon.

Put simply, I’m torn about this plot point. On one hand it will give us the single best ship shot of the entire season as Discovery swoops in on the Charon and destroys the mycelial reactor, but on the other it ascribes properties to the mycelial network that seem to come out of left field and that verge on the spiritual/magical.

According to Stamets, the death of the mycelial network will result in the death of “all life in all universes” but he gives no explanation as to how or why. Perhaps this was just a throwaway line meant to instill dramatic urgency, but left unexplained it opens up a pretty big can of metaphysical worms.

Back on the Charon, Burnham and Georgiou team up to spoil Lorca’s plans. Burnham brings Georgiou to Lorca and concedes to his demand that Burnham stay with him in the Mirror Universe, but on one condition: Discovery and her crew must be spared. Lorca agrees and sets himself up for what is, frankly, a pretty obvious double-cross.

Discovery fires on the Charon while Burnham and Georgiou fight Lorca and his lackeys. The battle is not only excellently choreographed and performed, but also a nice mirror to the battle aboard the Klingon sarcophagus ship in “Battle at the Binary Stars.” Yet again, Georgiou and Burnham find themselves fighting a fanatic, but this time both women make it out alive after vanquishing their foe.

After my lukewarm reaction to all the twists and turns this season, it was refreshing to see Georgiou and Burnham come up with a plan that hinged on mutual trust and actually have that trust be rewarded. Burnham didn’t lie to Georgiou about her plan to confront Lorca in the throne room, and Georgiou didn’t backstab Burnham.

The dramatic payoff was in watching two characters with a very complicated relationship finally understand and appreciate one another. Throwing in another confrontation between the two would only have muddied that.

Georgiou kills Lorca and intends to give her own life defending Burnham, but in the final seconds prior to being beamed off the Charon, Burnham grabs Georgiou, realizing she can’t be responsible for Georgiou’s death yet again. Materializing aboard Discovery, Georgiou is surprised and, if not furious, deeply concerned. Mirror Georgiou, the ruthless former leader of the Terran Empire, is about to take a trip to the Prime Universe and there’s nothing she can do about it.

Discovery destroys Charon’s reactor and begins her ride on the mycelial shockwave, but Stamets struggles to navigate the network. Eventually he gets the ship back home, but unfortunately he overshoots the temporal component of his coordinates by nine months. Attempting to contact Starfleet, Discovery makes a chilling realization: the Klingon-Federation war has ended, and the Klingons have won.

It looks like next week’s episode, “The War Without, The War Within,” will explain what’s been going on in the Prime Universe over the last nine months.

Did the Mirror Discovery have anything to do with the Federation’s defeat? Is Prime Lorca dead? And what’s Mirror Georgiou going to do with herself in a universe full of human idealists?

Star Trek: Discovery returns next Sunday for its penultimate episode of the season.