Enterprise travels to Terra Nova, a semi-utopian colony experiment from 70 years prior. It seems that a band of Earth colonists travelled 20 lights years to set up a colony away from Earth, then, after getting into a political dispute with the home world, cut off all contact. What happened to the colonists had been one of the greatest mysteries in modern Earth lore.
Upon arriving at Terra Nova, Enterprise discovered an abandoned facility and measurably high amounts of surface radiation. Archer, T'Pol, Mayweather, and Reed go to the surface to investigate and, when Reed comes in contact with a strange species, they track this new discovery into a series of Viet Cong-like underground caverns.
Reed is wounded and captured by what turns out to be the remnants of the Terra Novan colony. Trouble is, the Terra Novans think they are another species that is in conflict with Earth. The crew discovers that a radioactive comet decimated Terra Nova 70 years ago, killing all of the grown-ups and leaving the children to fend for themselves beneath the surface. Believing both that Earth was responsible for the comet and that they were different species, the young Terra Novans forged their own primitive culture.
But, when Archer and crew discover that the Terra Novans are still in danger from the radiation, Enterprise must convince the Terra Novans of their true heritage in order to save them.