Dr. Rodney McKay undergoes experimental gene therapy, altering his DNA to match the Ancient genetic coding. The experiment opens up countless scientific possibilities, as well as an unexpected problem. Meanwhile, during a game of hide and seek, one of the Athosian children inadvertently opens a containment unit that lets loose a seemingly harmless black ethereal cloud. But when Atlantis goes through a series of technical malfunctions, they realize that the shadowy creature is actually feeding off of the power supply!