Up the Long Ladder
"Up the Long Ladder" is an episode from the second season of Star Trek: The Next Generation.
The USS Enterprise-D receives a distress call from a human colony from the planet Bringloid V, which is in danger from solar flares from a nearby star. The colony turns out to have been founded by the crew of the SS Mariposa, a cargo freighter launched from Earth in the 22nd century and destined to the Ficus sector.
The colony, led by an Irishman named Danilo Odell, has reverted to a pre-industrial rural lifestyle. When rescued onto the Enterprise-D, the colonists are at first amazed by the 24th century technology on the ship, but they quickly adapt to it.
When the colony has been rescued, Odell informs Captain Jean-Luc Picard of another colony, also descended from the Mariposa, located half a lightyear away. The Enterprise-D also visits this colony, which has taken to calling itself Mariposa.
The Mariposa colony is strikingly different from the Bringloidi colony. The Mariposa colonists have kept their advanced technology, but when the colony was founded, there were too few survivors to establish a stable gene pool. This caused the Mariposans to turn to cloning instead. For almost three centuries, every Mariposan has been the clone of one of five original colonists, and now the colony is in danger of dying out because of reproductive fading: genetic errors will soon cumulate into a terminally fatal stage, killing the colonists.
The Mariposans ask the Enterprise-D crew for a sample of their DNA, so they could create new clones. The crew refuses, so the Mariposans kidnap Commander William Riker and Doctor Katherine Pulaski to steal their DNA. When Riker and Pulaski find out, they visit the colony's cloning labs and destroy the new clones.
Alarmed, the Mariposan prime minister Walter Granger once again turns to Picard and his crew. Doctor Pulaski tells them they need new breeding stock to establish a real gene pool instead of cloning. The Bringloidi colonists would serve as a perfect source for this gene pool.
The Enterprise-D crew and the members of both colonists agree to integrate the Bringloidi and Mariposa colonies. However, monogamic marriage will at first be temporarily suspended, to ensure fast development of a healthy, sustainable new generation. Thus each man will have three wives and each woman will have three husbands.