In 1991 a flying saucer crashes in the Mojave Desert containing a race of extraterrestrials, the Tenctonese, escaping from slavery under a cruel Overseer race. They resemble humans, but have certain anatomical differences and have been bred with greater physical strength and intelligence. These Newcomers, as they are called, are accepted as the latest immigrants to America, and the franchise explores issues around their integration into the multicultural society of the US.
The year is 1991; three years since an unidentified flying object bearing 300,000 enslaved aliens, the Tectonese, landed in the Mojave Desert on planet Earth. Los Angeles later becomes their new home.
"That was the scene in California's Mojave Desert five years ago - our historic first view of the Newcomers' ship. Theirs was a slave ship, carrying a quarter million beings bred to adapt and labour in any environment. But they'd washed ashore on Earth, with no way to get back to where they came from. And in the last five years, the Newcomers have become the latest addition to the population of Los Angeles... "
In a recapitulation of the series' cliffhanger, ''Alien Nation: Dark Horizon'' begins with Susan Francisco and her daughter Emily falling victim to a newly developed viral infection that was created by a group of human Purists to exterminate the Tenctonese species. There is also a new sub-plot running parallel to this one, the story of Ahpossno, a Tenctonese Overseer who lands on Earth to find any surviving Tenctonese and bring them back into slavery. The idea of a signal sent into space by the surviving Overseers was explored in the ''Alien Nation (TV series)'' episode "Contact".
It follows the television series format of two parallel storylines. The first plot is about a seemingly human-Tenctonese Hybrid (biology) child involved in a sinister experiment with a Newcomer scientist disguised as a human. The sub-plot is the budding relationship between Matt Sikes and his Tenctonese neighbor Dr. Cathy Frankel. The relationship between Matt and Cathy was an ongoing theme of the ''Alien Nation'' television series.
The plot follows human detective Matthew Sikes and his Tenctonese partner George Francisco as they investigate a mind-altering Tenctonese artifact being used to lure followers into a deadly cult. The artifact used in this film was the same one from ''Alien Nation (TV series)'' episode ''Generation to Generation''.
Detective Matthew Sikes and his Tenctonese partner George Francisco investigate a group of Tenctonese called the Eenos. The subplot involves Tenctonese binnaum Albert Einstein and his new bride May attempting to have a child (with George's help) -- a reversal of the ''Alien Nation'' episode "Three to Tango".
The plot introduces the idea that among the Tenctonese slaves, there was a resistance movement called the Udara who were implanted with hypnotic suggestions to act as sleeper agents. Now, here on Earth, someone has found a way to activate these sleeper agents, and send them out as Assassination.