S.H.I.E.L.D. calls upon the Avengers to travel to Hydrobase, a free-floating island of scientific research owned and operated by the Roxxon Corporation. S.H.I.E.L.D. satellites are reporting massive energy discharges from the base, but because it operates as part of a multinational company, S.H.I.E.L.D. cannot investigate. Iron Man, Hawkeye, Captain Marvel, Black Panther, and Scarlet Witch travel by Quinjet to Hydrobase, where they are met by the base’s heads of research, Diane Arliss-Newell, and her husband Doctor Walter Newell, who has created a super-powered exoskeleton for underwater research and calls himself Stingray. They are soon joined by Lucino Nefaria, who arrives by helicopter with his bodyguard, the superhuman mercenary called Paladin.
Nefaria orders Paladin and Stingray to attack the Avengers, while he retreats into one of the research buildings. Iron Man and Captain Marvel dive below Hydrobase where they realize they cannot find any leaks, but odd energy IS radiating through the containment walls of the facility. They are suddenly attacked by a group of blue skinned aquatic humanoids – Atlanteans – led by their ruler, the human/Atlantean hybrid Namor, the Sub-Mariner.
The Avengers best Paladin and Stingray easily, and the Panther follows Nefaria, finding a chamber where scientists are experimenting upon a strange mystical artifact – The Serpent Crown. The Panther incapacitates the scientists and defeats the chamber’s guardian, Tiger Shark.
On the surface, Namor battles the Avengers, being finally beaten by Hawkeye.
Nefaria manages to escape via a submersible escape pod. The Avengers turn the Serpent Crown over to the Atlanteans, with Namor warning them that future interference into Atlantis by the surface realm will be considered an act of war. The Panther also vows to bring charges against Roxxon for environmental crimes.