Black Panther, Captain America and Iron Man are enjoying some down time in Avengers Mansion when a man in green armour attacks them. His weaponry is impressive but he seems only to be using it to get them to fight him. He urges them on to kill him, saying he won't stop until they do. But Iron Man blasts him until he can't stand up..
When they remove the helmet the find industrialist Jason Beere, who claims to be dying - his heart is failing. But when he goes he wants the world to go with him. And then he passes out. Shellhead takes him to the Stark factory in Flushing, Queens, where he hooks him up to 1 of his old chestplates to keep his heart beating.
When Cap and Panther catch up to him, Iron Man tells them that Beere has planted 4 neutron bombs around the world that will explode when his heart stops. He only has months to live. The bombs will be enough to wipe out all life on Earth.
Later we learn this info was found in a diary and notes in Beere's safe, so IM must have gone to get this. But they only know where 3 of the bombs are, not the 4th that will trigger them. But the notes showed that the bombs could be disarmed by fitting them together. They pick 1 bomb each to find, and hope to be able to work out where the 4th is.
Cap flies a quinjet to Peru. He easily locates the bomb in plain sight in a temple pyramid. But he is attacked by native cultists of a bird god who have been paid to guard it. They even seem to expect him. The leader sets harpy eagles on Cap, but he avoids them by kicking a bee nest at them. Then he grabs the bomb and runs.
Panther finds his bomb in an Arctic cave, but has to get past a polar bear. He succeeds without harming the creature beyond stunning it.
Iron Man has to retrieve his bomb from a Moscow museum. He sneaks in under the radar, but the Soviets too have been alerted to expect him. He battles past guards and security systems to get hold of the bomb. But then he talks his way out, and avoids an international incident, by showing them that it's a very dangerous bomb he's removing from their city.
When all 3 Avengers are back in Stark's plant, Iron Man fits the 3 bombs together as Beere's instructions described. He has examined the bombs and confirmed that they have no detonators. He has also figured out what they will do when connected, and so isn't surprised to hear a recorded message from Jason.
Beere's notes had been planted for the Avengers to find. The quest for the 3 bombs was just to distract them from looking for the 4th detonator bomb. He claims that it is now too late to find and disarm that detonator.
But Iron Man has worked out where the 4th bomb is - it's inside Jason Beere, attached to his heart. He checks and finds this is true, but can't figure a way to remove it without setting it off.
However he has a plan. He puts Beere in a cryogenic chamber. The extreme cold will slow his heartbeat down a lot. And the chestplate will keep it going for longer. So now Tony Stark will have years to figure out what to do about the bomb.
