Synopsis
Last issue ended with Sub-Mariner's attack on the Avengers in Wakanda with a tidal wave and his Atlantean troops. This issue describes that attack.
Lei Kung had created a portal to K'un Lun, and Hope Summers and Wolverine went through it. Now Iron Man takes Lei Kung to follow them, telling him to close the portal behind them, lest the Atlanteans find the way. His parting words to Captain America are to use their 'secret weapon'. Black Panther and Iron Fist are left behind with some Wakandan spear carriers (well, advanced weapon carriers).
Cap heads off with the others, supposedly to talk sense into his old friend Namor (but he never gets much chance to do that). Along the way he points out that this attack on innocent Wakandan civilians with water and sea-monsters ridden by Atlanteans proves that the X-Men are the villains of the piece, and therefore the Avengers are the good guys.
Cap baits the flying Sub-Mariner to come down and attack him, and then he sends out the call for all Avengers to converge here. Meanwhile Iron Fist and Black Panther tackle Namor, while Cap faces a hulking Atlantean warrior. Sub-Mariner KO's Iron Fist, but merely trades threats with Panther. But that holds him long enough for a horde of other Avengers to descend on him (Beast, Daredevil, Dr Strange, Falcon, Quicksilver, Red Hulk, Spider-Man, Thing, Thor, Valkyrie and Vision).
The assembled cast commence pounding on Namor, who doesn't inflict any significant counter-blows except to break Red Hulk's arm. (Some Avengers don't do much, including Dr Strange.) The battle boils down to a fight between Sub-Mariner and Thor, which Namor seems to win. Then Cap calls in Scarlet Witch for a duel between Phoenix Force and her Chaos Magic (which last issue we learned were somehow connected). The effort exhausts Wanda, but Namor is down too. (Wanda is rescued by her ex-husband Vision, who as of last issue definitely hadn't forgiven her for killing him during Avengers Disassembled.)
The other Phoenix-infected X-Men (Colossus, Cyclops, Emma Frost and Magik) have been watching the events with Magneto in Utopia. Now the 4 of them arrive via the Siege Courageous, and the Phoenix Force abandons Namor and shares his portion between the others, increasing their individual power.
Cap chooses this moment to use Iron Man's 'secret weapon'. A device on his wrist reopens the portal to K'Un Lun, and the Avengers stagger through, reuniting with Hope Summers' small gang. The X-Men are left to face a mental vision of Professor X, who asks them to stop, or he will have to stop them.