Synopsis
We start with a flashback (2 of Rick Magyar's pages of New Avengers #6 very faithfully redrawn by Jim Cheung) where the Illuminati and Black Swan destroy an alternate Earth over Latveria. That's all we see of them this issue except for Black Bolt and Iron Man.
Now Thanos holds court on ruined Titan where he and Corvus Glaive receive news from an Outrider assassin. He claims to have had success on some mission to the planet Ahl-Agullo. But he is sent to complete his mission (which involves 'making known what remains hidden' and 'finding another world to raze').
Meanwhile Glaive goes to Ahl-Agullo, which Thanos's forces decimated 2 cycles ago. Since then the huddled remnants have rebuilt a bit of civilisation. Glaive reminds them how he killed their champion, the king's son. He says Thanos now demands a tribute, or they will be smote again. Glaive returns to Titan with the tribute - a box full of heads, to add to the skulls that pave Thanos' court.
The Outrider comes to Earth to search for what is hidden. In stealth mode he eavesdrops on Avengers (Captain America, Iron Man and Thor), X-Men (Kitty Pryde, Storm and Wolverine) and SHIELD, but doesn't yet find it.
We now skip to Galador where we left Captain Universe in Avengers #16. The Spaceknights' home planet was under attack. It turns out the attackers are the Builders who have been mentioned in most of the Marvel NOW Avengers issues. The Spaceknights defence is in vain, and CU sees Galador destroyed.
Abigail Brand of SWORD is overseeing the capture of some Skrulls on Earth. Captain America and Hawkeye are accompanying the troops, with Hyperion and Thor standing by in case they're needed. But the Skrulls turn out not to be warrior caste, like the other refugees SWORD has intercepted. (This bunch slipped past SWORD in Av#15.)
Captain Universe materialises in the Peak, in her mortal form in a coma. SWORD gets a transmission from a Kree outpost being wiped out. Ex Nihilo, who joined the Avengers in Av#17, recognises the attackers as Builders, and a fellow Gardener (different from one we saw on Galador). Adding this attack to the stories of the space refugees who arrived in Av#15, Brand calculates that the Builders are heading for Earth.
The Outrider has come to Attilan where he invades the sleeping mind of the Inhumans' king Black Bolt. He discovers lots of stuff:- The Kree created the Inhumans. They get their powers from the Terrigen Mists. Black Bolt, married to Inhuman Medusa, took other wives as a political manouver from 4 alien races - the Universal Inhumans, more results of Kree tampering (as seen in Hickman's run in Fantastic Four). His brother Maximus the Mad has secretly been building devices for him (as seen in NAv#7&8). Black Bolt is a member of the secret Illuminati. (These last 2 facts have caused dissension between BB and his wives.) The Illuminati had the Infinity Gems (as seen in Illuminati #2), but lost them (NAv#3). But even this isn't what Thanos is seeking.
The Outrider finally finds what he seeks. We see an image of 4 kings and a queen. One of the men wears the crown of Attilan.
At this point Black Bolt wakes up and tears one of the Outrider's 4 arms off. The Outrider makes his escape, but is blasted by a whisper from Black Bolt. However he falls onto his invisible spaceship, and completes his escape.
Captain Marvel addresses the assembled Avengers, including Abyss, Ex Nihilo, Nightmask and Starbrand recruited for this emergency in Av#17. Word from their Shi'ar representative Smasher is that the Galactic Council is mobilising a defence against the Builders. Abyss confirms that her creators the Builders are fully capable of destroying the Earth if they get here. So Captain America says that most of the Avengers will leave Earth to help the Council. Iron Man will stay behind on Earth to coordinate local defences.
The Outrider reaches Titan to find that Corvus Glaive has been joined by the rest of the Black Order:- Black Dwarf, Ebony Maw, Proxima Midnight and Supergiant. He reports that Black Bolt hides what Thanos seeks, and then gratefully accepts the reward for a job well done, as Glaive chops his head off. Glaive announces to Thanos' minions that they will invade Earth. He quells their mutterings of discontent when a mole from SWORD tells them that the Avengers have left the planet.