This is an alternate-future story involving the far distant future of the Fantastic Four, and virtually everyone in the Marvel Universe.
Sue Richards with the help of an elderly Dr. Strange and his daughter, has acquired an ancient Kree artifact from the Earth's core, snatching it away from the Mole Man and his minions. Now, reunited with her husband, Reed Richards and her fellow teammates (The Human Torch and The Thing), she is determined to get them to help her in her quest to rescue her children from death in the past at the hands of Dr. Doom. Meanwhile the Avengers and friends (Spider-Man, Daredevil, and Luke Cage et. al.) join the Shi'ar's Imperial Guard as they confront confronts Ronan in the Negative Zone and Nick Fury and SHIELD operate an intergalactic Helicarier outside the orbit of Pluto.
Sue is trying to get the rest of the team on board with her plan, but they all think that the kids are dead, even though Sue insists that they are not. While Sue brings her teammates around, The Avengers and Co. are thrashing Ronan's hoards. As it turns out, Sue was right, and Franklin and Valeria didn't die, but were, in fact, snatched at the last minute by the FF from the future. As the heroes clash with the Kree armada, Galactus arrives and declares that none may harm Earth without invoking his wrath, and the fleet scatters.
Meanwhile the FF battle Doom, and rescue the kids, with Doom himself winding up falling through the gateway into the Negative zone, getting trapped there forever. The FF wind up hooking up with the Avengers, the Inhumans, SHIELD, and the rest of Earth's heroes showing that all's well that ends well. Not the worst of the lot, but I'm still trying to figure out the point of it all.
Having not read the initial chapters of this series, it makes it hard to completely follow what is going on in this story
Again, since this is one of those epic (pre-Civil War) battles that require everyone's presence, our pal Spider-Man is there because, well he's Spider- Man, and it's like a rule.