Synopsis
This issue begins with a reprise of #1, ending with the arrival of the Illuminati in Wakanda. Last issue we learned that T'Challa is King of the Dead in the Necropolis, while his sister Shuri still rules the living. (Presumably both still use the title Black Panther, but Shuri's not in this issue, so there won't be any confusion.) Sub-Mariner is a war criminal in Wakanda since Avengers vs X-Men #8, so Panther has him (and possibly the other Illuminati) hidden in the Necropolis without the queen's knowledge. (Does she even know about the problem in this story arc?)
The Necropolis also holds captive the Black Swan from last issue. Reed Richards comes to interrogate her. He notes that she has an unknown device implanted within her. He establishes that the team she led came from another Earth - an Earth that briefly hovered near our own until she caused its destruction. But she suggests that that particular Earth wasn't her home. Mr Fantastic asks why she killed one of her team called Manifold. She replies that he was no longer any use because his abilities didn't work outside his own universe.
But Reed also comments that the security cameras show that she has been crying, and apparently praying in Sumerian. At that she breaks down and claims that she only does what she has to do. And he doesn't understand because he doesn't know what's going on. After that she presumably tells Richards everything.
T'Challa goes to visit Namor, who is guarded by some Dora Milaje. They are uneasy about keeping his presence secret from Shuri and the other Wakandans. Panther reminds Sub-Mariner that he must remain hidden at all times. Namor taunts T'Challa that he refused to join the Illuminati, and now he needs their help. Panther tells Namor that when this is over he'll hunt him down and kill him.
They join the other Illuminati (Black Bolt, Captain America, Dr Strange and Tony Stark) as Mr Fantastic explains what he has learned from Black Swan.
During #1 we heard certain words being used by Black Swan and her team, and they were repeated in the reprise and Swan's rant. She said that she wasn't Black Swan, but a Black Swan. The event was called an incursion. The murder of Manifold and the destruction of the other Earth were described as sacrifices to the Great Destroyer or the Wheel.
Reed Richards' lecture begins with a page from last issue where he said 'everything dies'. But he continues by saying that they have been gathered here to stop everything dying prematurely. Dr Strange says they've faced the end of the world many times before. What's so special this time that made Panther call on the group he despised. T'Challa says that the future of his people died yesterday (presumably referring to the deaths of the 3 brightest Wakandan youths in #1), and that what followed was even worse (the destruction of an Earth).
Before they continue, Namor wants to make sure there are no ringers in the group. (He's presumably harking back to when Black Bolt was replaced by a Skrull before Secret Invasion.) He's brought his Infinity Gem along, which can be used to detect lies. Reed Richards and Steve Rogers have brought theirs too, but Dr Strange and Iron Man have left theirs securely stashed. The group reckon that 3 Gems should be enough to detect a fake, and they each state their names. (Stark speaks for Black Bolt, whose voice would destroy the building, but that seems OK.) The revelation that the Illuminati have the Infinity Gems only increases T'Challa's conviction that such a group of secret masters is a bad idea.
Finally Mr Fantastic explains what's going on.
He briefly describes the birth and future death of the universe, and the birth and death of the Earth within that span. He brings in the alternate universes, with their alternate Earths. All universes heading for the same end (which for the purposes of the plot is shown as a single end point that all the alternates are converging on, rather than each universe having its own end).
But Reed then claims (presumably from Black Swan's story) that an event caused one of these alternate timelines to end abruptly. This caused the end of all timelines to come a little bit closer. But this shrinking then caused 2 adjacent universes to collide, and mutually destruct. Which shrank the future some more, causing more destructive collisions. And the process is accelerating. (There is no indication that the colliding timelines are near the original problem universe.)
Mr Fantastic says that the point where the universes touch is called an incursion. And because the event that destroyed the 1st timeline was on an Earth, Earths are the sites of the incursions. One of which Panther witnessed in #1.
The conversation turns to how they might handle the situation. Tony goes into engineering mode. Can they discover the cause of the problem? Can they find a way to stop or at least slow the chain reaction? Reed mentions that they have the device Black Swan used to trigger the destruction of the other Earth. Captain America stamps hard on that line of thought, obviously not willing to contemplate destroying planets.
But Mr Fantastic now gives them another layer of information. Before the incursion starts there is a short period of harmonic alignment, which they should be able to detect giving them advance warning. However, once the other planet appears, they have 8 hours before the 2 timelines self-destruct. The only way out is to destroy one of the Earths within that time.
Dr Strange suggests that they need to think about saving not just our universe but all of them. (For 1 thing, if the multiverse expires early, so does our universe.)
But Sub-Mariner brings them down to Earth by asking who would be willing to kill another world to save ours (although he doesn't quite say that). Black Bolt responds with an extremely quiet noise which cracks their stone meeting table (but as usual without Medusa to interpret it's difficult to grasp his meaning). But Tony Stark is clearer when he proposes that the imminent end of everything means every solution should be considered.
Still Cap is adamantly opposed to such a course. Instead he proposes using the Infinity Gems. (No-one reminds him that he refused to countenance such an idea in Avengers (2010) #7-12.)
Mr Fantastic points out they'll need all 6 Gems. Dr Strange reminds them that the Mind Gem's location is unknown since Prof X died. But Namor counters that they can use the other 5 Gems to find the 6th, since (as mentioned in the aforementioned Avengers story) the gems call to each other.
They immediately set a plan in motion. Stark, Richards and T'Challa will start building the incursion early warning system. Namor, Black Bolt and Cap will go after the Mind Gem. (No-one mentions that Iron Man and Dr Strange will have to go get their Gems 1st. Nor what Strange will be doing afterwards.)
Cap then gives an inspiring speech, but behind it we see scenes of carnage. These appear to be future scenes (3 have another Earth in the sky). We see Mr Fantastic manipulating some energy while Iron Man (minus an arm) defends him from monsters. We see Black Bolt shouting while surrounded by enigmatic figures. We see Sub-Mariner killing a possible super-character while surrounded by others live and dead. We see Black Panther and Dr Strange battling armoured foes.
Later Reed and T'Challa discuss the situation privately. They think big. Richards is of the opinion that if the overall problem were easily solved, someone in the infinite multiverse would have solved it already. He also sees 2 possibilities. Either the effect is built-in to the multiverse, and so can't be permanently corrected. Or someone is causing it to happen.
They agree that if Cap's plan doesn't work then they're going to have to go the Black Swan route, and become destroyers of worlds.