S1, E3: Dr. Rodney McKay (David Hewlett) 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!