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!
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Complete story — includes the ending
Eager to experiment with new technology, Dr. McKay volunteers to be Dr. Beckett's first patient for his new ATA gene therapy. Now able to use a personal shield emitter, McKay recklessly tests its properties, only to find that it could lead to his death. Elsewhere, a "living shadow" entity threatens the city after it was accidentally released from a container where the Ancients were studying it. It roams the city looking for energy to feed on, and the more it feeds, the more lethal it becomes. Finally members of the Atlantis expedition manage to send it to planet M4X-337.