“What’s Past Is Prologue” is episode thirteen of season one of Star Trek: Discovery.
In the previous episode, “Vaulting Ambition”, Burnham, whilst talking to the emperor, realised that the Lorca she knew was actually the Lorca from the Mirror Universe. Lorca himself was currently escaping confinement, thanks to someone’s desire for revenge.
This episode opens with Lorca’s people from the Mirror Universe being freed from their agonisers. That’s a problem with deciding to torture your enemies, not kill them; they are still alive. Lorca is telling his followers that he has returned and that they will reclaim their empire. One of the people is Mirror Landry. Landry says that there are 18 battalions of imperial guards on the ship; they need to redraw and regroup. Not according to Lorca. The emperor’s ship is where they need to be. He’s been to another universe and back; did she think he’d have come back without a plan?
They head to the lab of Mirror Stamets, Stamets having managed to get both himself and his mirror version out of the mycelial network. Lorca says that if there’s one thing the Stamets have in common, they love their work, and wouldn’t have left any of it behind. Grabbing Mirror Stamets out of a hologram. Stamets really hoped Lorca was dead. He’s told he can help finish what they started, before he sold them out and betrayed the coup. Stamets wants to know how Lorca is still alive. Lorca’s ship, the Buran, came under attack and entered an ion storm just as Lorca was beaming aboard. He ended up in a parallel universe. Stamets believes the ion storm must have swapped the transporter signatures. Stamets was working on a bioweapon; Lorca wants it. And uses it.
The emperor is not impressed, though Burnham warns her about Lorca manipulating her. He wants the emperor to come to him. Burnham also wants to contact Discovery, because they don’t know what they are flying into. However, the emperor isn’t going to let Burnham be her weakness again and orders her to be taken to the brig. Something Burnham doesn’t cooperate with and escapes. Georgiou wants Owosekun to bring Lorca to her so she can kill him herself.
On the Discovery, Stamets is better again which means they have their spore drive. However, during his time in the network, the ghost of Dr Culber told him that Mirror Stamets had corrupted the network. If they can’t save the network, it would be bad. The central orb of the Charon is mycelial in nature and is pulling power out of the network, more than Stamets could have believed. The network normally regenerates itself, but it’s been poisoned. Without fresh supplies of spores, it will get worse. The Terrans weren’t bothered that it wasn’t sustainable. If it isn’t stopped soon, the network will deteriorate everywhere, across every universe, ending life.
Lorca is broadcasting to the Charon why he is so much better than the emperor for the job, trying to convince the crew to renounce her. He also wants Burnham. Lorca has been traced, so Georgiou takes her troops after him personally. They meet Owosekun on the way, who says they were ambushed and the rest of her troops killed. She was spared because Lorca wanted the emperor to know he was there. Then she’s vaporised. The emperor’s people are3 behind a containment field and she uses automated guns to take down many of Lorca’s people. The guns, however, are taken down and the containment field weakens and the emperor emergency transports out.
Burnham manages to contact the Discovery and fill them in. Stamets realised that Lorca altered their last jump, that the attack on the Klingon ship in “Into the Forest I Go” gave him everything he needed. If they directly hit the Charon‘s energy orb, it will sever the connection. However, the containment field needs dropping first. Burnham will deal with that.
The big problem with the attack on the Charon is it seems to be a suicide mission; success will mean their destruction. Burnham has to make a deal with the emperor in order to deal with Lorca. Lorca himself is rather fixated on Burnham. That might be a weakness on his part.