Synopsis
Thanos has been elected Prime Eternal with the help of wily Druig, but is trying to find out how to shuck his Deviant heritage and become a genuine Eternal to be fully accepted by the Great Machine. The heroic Eternals we tend to concentrate on are trying to find out how to defeat him by sneaking into Avengers Mountain HQ to interrogate the dead Celestial whose body comprises the HQ. It hasn't gone well. As usual the Machine provides voiceover comments.
Some of the Eternals (Gilgamesh, Ikaris, Thena) are fighting some Avengers (Captain Marvel, Iron Man, Thor) in a place called Little Hollow, but it's mainly a ploy to keep the Avengers away from their base. In the HQ Kingo Sunen is duelling Black Panther, and Sersi and Sub-Mariner are keeping each other occupied in a hot tub. Meanwhile priestess Ajak has been questioning their god with help from Makkari. But what she has learned is a shock - the Deviants were more important to the Celestial plan than the Eternals were.
An info-dump/diagram explains it all. 4 billion years ago the sick Celestial called Progenitor died on Earth and his 'necrofluids' infected the world. This resulted in occurrences of unstable strange lifeforms which never survived. 1 million years ago Celestials created the Deviants to interact with the necrofluid. They stabilised it but were themselves genetically unstable. The Eternals were created to cull the Deviants of Excess Deviation, but only so that they can continue their purpose. They interbred with other Earth lifeforms to produce stable strange lifeforms, culminating in the Age Of Marvels (who were able to defeat the Celestials' enemy the Horde at the beginning of the current Avengers series).
Priestess Ajak had always believed that the Celestials were gods, but now she realises that was all part of the Eternals' programming. She thought the Avengers blasphemers for using the Progenitor's body as their home, and that the Celestials would return to them if they ejected the heretics. But now she realises that the Avengers are just as much heirs to the Celestials as the Eternals are. Now her anger makes her want to see the Celestials brought to answer for their actions.
Thena mentally contacts Ajak to see how things are going. Ajak admits that she has also found what they need to kill Thanos, and she uses the Machine to transport her Little Hollow to help in the fight. She punches out Thor who had Thena on the ropes. But then another Avenger rejoins the fray. Last issue Phoenix (Maya Lopez) was trapped in a space-time bubble. But now she breaks out and uses her Phoenix-fire to create a bubble of her own around the combatants, including Makkari who's super-sped here.
In Olympos Thanos and Druig have been working their way through the mind of the Titan's father A'Lars/Mentor to find out how Thanos was created. Now they find the design schematics for the being and leave the wrecked mind. Domo uses this new knowledge in the resurrection engine, but still fails to make the Titan a true Eternal. Domo says that Thanos was made with the necessary connectors to integrate with the Machine, but now they are jammed with something he doesn't understand. "Death lies congealing in every part of you."
Thanos laughs. He has changed himself, something the Eternals could never do. In his mind this makes him better than them all. In this mood he gives the failure Domo a swift death (but anyway the Eternal will just be automatically resurrected). Then he reminds Druig of his backup plan. Since he can't become a full Eternal he'll just destroy the Great Machine (and with it the Earth). In the Et: The Heretic 1-shot eternally imprisoned Uranos gave him the location and passcodes to his ancient armoury. Now he tells the Machine (which must obey the Prime Eternal) to activate Uranos' doomsday fail-safe which will do what he needs. Then he and Druig, along with the Oceanic Watch team of Eternals go to the ancient ruined Eternal city of Titanos wher they can escape into the timestream.
Druig wishes he'd thought of doing something to prevent this. The Machine can't disobey but it can and does alert the Eternals. Sersis telepathically tells the others to stand down and asks them to link her telepathically to the Avengers. She informs them that Thanos is alive and on Earth and about to destroy it. She begs them to let her people go and stop him. Captain America is with Black Panther and Kingo Sunen and tells his team to comply. CA and BP let KS go. Phoenix drops her bubble and the others go ...
... to Titanos where Kingo and Sersi join them. Thanos greets them and sends the Watch to battle them while he himself takes on Ikaris. The Eternal is no match for the Titan who then turns on Druig, suspecting him of telling his foes that he was leaving via Titanos. The Machine announces that it is priming the detonation ...
... but then it does something else. Thanos is shocked to see his limbs becoming encased in something. The Machine announces that it has been recently programmed to activate the fail-safe that Phastos programmed into Thanos' rebuilt body (#7) if the doomsday countdown ever got this far. And it reads a note from the programmer Druig relating that he set this up before Thanos, as he expected, wiped knowledge of the fail-safe from his mind (#8). Now Druig gloats over his (forgotten) cleverness as Thanos is completely encased. And he kicks him into Titanos' unstable time-slices. The Machine can now halt the countdown, and Druig claims the victory as the other Eternals are still fighting the Oceanic Watch.
Sersi explains to the assembled Avengers what has been happening in more detail. Tony Stark is angry that they kept the danger to Earth a secret. She gives him the standard Eternal reply that it's been their planet much longer than humans have been on it. And they've been charged by the Celestials to look after it. But then she adds a personal note that they are right not to trust her people. Thor broaches his anger too, but Sersi reminds him that *they* were exercising their responsibility when his father Odin was a drunkard. Another Eternal, the ever-young Sprite, pops in to calm things down. She's accompanied by the even younger Avenger Starbrand who she's been keeping amused during the conflict.
The Eternal duo meet the others in Celestia to assess the situation. Thanos is gone but their relations with the Avengers have been soured. Now that they know new stuff about the Deviants their scientists may be able to help that race. But Phastos is in the Exclusion Zone (but surely they can now get him out), Gilgamesh has left (as he always does). And now Ikaris gets called away ...
... to go see Sophia Robson, the mother of the boy Toby he failed to save. But she only wants to praise him for saving Little Hollow (last issue). He still can't tell her that Toby died to fuel his latest resurrection (#6).
In an epilogue Oceanus (the brother of imprisoned Uranos and currently dead Zuras) proposes Druig for the next Prime Eternal. He just saved the planet while bringing Uranos' armoury under their control. They can easily depose him if they don't like what he does. He will provide stability after the things they learned about themselves (from the Celestials' Final Host in the 1st arc of the current Avengers series) and now about the Deviants.
Now Druig sits on the throne and thinks that to cement his rule he needs to give the Eternals a purpose again. He asks the Machine to locate areas with large amounts of Deviant DNA. As expected it highlights their city Lemuria. But also the island Krakoa, home to the mutant nation. (It also adds that they've now expanded to Mars.) Druig's face lights up. This is surely an example of Excess Deviation, which the Eternals are programmed to root out.