Synopsis
The 'previously' page shows the team using the Infinity Gems to terminate an incursion by another universe in #3, but at the cost of losing the Gems. On the next incursion last issue they set out for the other Earth to find that universe's copy of the Infinity Gems. Such Gems could only be used in their home universe, so 1 of the team would have to stay in the new universe to operate them. However the point is moot as they find Galaktus about to solve the problem by destroying the alternate Earth. Being heroes the team fight his herald Terrax to try to stop him. Now read on.
Terrax makes short work of most of the team, but Iron Man manages to hold him off until Black Bolt KO's him with a whisper. However they have to abandon that world to Galaktus, and take Terrax back to their own Earth. Where he is now a resolutely mute captive.
A week later, in Black Panther's Necropolis in Wakanda, Reed Richards is again interrogating the Black Swan whom they captured in the 1st incursion of an alternate universe. She has already explained the basic mechanism of incursions in #2. The 2 universes always intersect at their Earths, and both universes will self-destruct unless 1 of the Earths is destroyed within 8 hours. We saw her destroy such an alternate Earth in #1. The Infinity Gems allowed the more humane option of separating the universes.
Black Swan is as usual full of cryptic utterances. But she promises to be more forthcoming in return for some freedom. Black Panther lets her out of her energy cell, but with a bomb necklace as a safeguard.
Swan refuses to talk to the 'necromancer' Dr Strange, much to the amusement of Sub-Mariner. But she recognises Beast as a frequently-important figure in other timelines' dealing with incursions.
She tells them of her history. She starts with a seemingly unrelated catastrophe which left her Earth without any sun or stars. She was Yabbut Ummon Tarru of the royal family, and her brother the heir Dadingra held the Great Key to the Library of Worlds. He never went further than opening the door to the Library, because he would get lost in the paths between worlds. Only the fabled Great Ladies could navigate the paths.
One day the sky turned red and an incursion brought the Black Priests from their Earth to destroy Yabbut's world. Only Yabbut escaped through the Library door. Inside she found 3 Great Ladies, Black Swans who raised her to be one of them. They taught her that the deaths of the timelines began when Rabum Alal, the Great Destroyer, was born. The Black Swans save universes by sacrificing Earths to him. The Library of Worlds has since been destroyed, and the Swans dispersed to do their work separately.
Our Swan mentions 1 method often used to keep a timeline safe:- evacuate Earth and destroy it. She refers to this as 'Shading the Apocalypse', the Eighth Way. And offers to tell them of other ways. But Panther doesn't trust her, since he saw her destroy an alternate Earth in #1.
Black Swan offers proof of her good intensions. The team have devices in their palms which alert them when an incursion starts. She says she can show them how to build devices which will give them earlier warning. She has such devices in her eyes. And they tell her an incursion is due in minutes. And it turns up in Latveria.