When various plants and animals wash up on the shores of the Amazon River, Martin & Diana are sent in to investigate. They uncover that digging by a hydroelectric company has released a strange green mist. Along the way they bump into Martin’s nemesis, Marvin.

Marvin discovers the secret of the two step siblings and that they're actually visiting the Amazon for The Center. Later, it's discovered that the green fog is actually an ancient bacteria that feeds off of oxygen, and has been suffocating other organisms in the area. It evolves, and appears after Diana lights a fire in a mold filled cave, in hopes that the fire would provide competition for the vapor and it would come into the cave, where the mold would destroy it.

It arrives however, with a new form after evolving, gaining eyes and tentacles, and traps the three in a cave, although thanks to some quick thinking from Martin, they're able to escape and the vapor is sealed inside the cave, being left for the center to deal with later.