The concept of the "Saeculum" refers to the lifetime of a population - the time that has to pass for a population to completely renew itself, with the idea of a certain civilization being given a certain amount of saecula in the background. The protomolecule-creators once had a civilization spanning the ring-gates, and centuries ago, they sent the samples that would later be found on Phoebe, and create all that has passed in the last years. The creators themselves are dead, but bits and pieces of their civilization exist everywhere - some of them in Miller, who has been tasked with finding what has destroyed the creators themselves.
"I reach wherever I wanna reach motherfucker"The Miller that now reappears in front of Holden is one that has fought back against what the protomolecule has turned him into. He retains aspects of who Miller used to be, and instead of just being the Investigator, the creation of the protomolecule-makers, he is his own man, and he has his own mission. He carries the voices of Eros in his head still, those millions of people who are perpetually screaming, and all he wants is peace and quiet. The only way to accomplish this is not just to find what has killed the creators, but to finish the job. And he has identified a dead spot right within the alien structure, a place that his mind cannot go to, a black hole of existence. This is where he sends Holden, to find his answer and to finish his mission once and for all.
This mission takes place while the Rocinante faces off with Edward Israel's bomb-wired shuttle, while Murtry goes off to retain his planet. Murtry is exactly what Drummer described in disgust in the previous episode, the kind of man that would enter a land to steal it, claiming terra nullius, claiming that the only way to be a man of the frontier is to be cruel and violent. He will do whatever it takes to stake his claim to the riches of Ilus, but he and his second-hand woman Chandra have no idea what they are up against. Chandra, who I haven't mentioned at all in this recaps, has built a relationship with Amos, falsely thinking that he is the mirror image of Murtry who once saved his life, having no concept of how far Amos would go to protect his family, the people that guide his moral compass. In the end, Murtry doesn't stand a chance against any of them, and all of his arguments that he is the man that the frontier needs, and Holden is what follows after civilization has been established, fall flat. His idea of a frontier is outdated, the whole conflict playing out beyond the ring gates is nostalgia about the past, in which the distinction between Belters and Inners mattered - out here, all that still has any currency is who will adapt fastest to the alien worlds that lie ahead, and who will comprehend first that there is a danger that goes way beyond the blinding microorganism or the neurotoxin-carrying slugs. When they speak of what happened here, Miller and Holden both realize what it means that the creators of this awesome network, of all these civilisations on all these planets, were wiped out - what it means that humanity is now deciding on using a network that was created by something that was wiped out completely. It will be a problem to awaken whatever thought it necessary to wipe out the creators of the protomolecule.
In Saeculum, that something is like an angry eye of god, nothingness waiting to be fed with the remains of the protomolecule. Miller is like a question trapped without an answer, an unfinished line of code waiting for the final command. He wants to extinguish himself, and every trace of the protomolecule that remains on Ilus, but in the end, it isn't Holden who throws him into the nothingness of non-existence, it's scientist Elvi, who once said to Holden that she would give anything to commune with the alien lifeforms, to ask for answers and comprehend what has happened. She doesn't understand the burden that Holden carries in Miller, but maybe now that will change. In the end, Murtry and his agenda is nothing but a footnote, something utterly unremarkable, an old narrative that never mattered on this new world, in this new paradigm of human existence.