"Up the Long Ladder" is an episode from the second season of Star Trek: The Next Generation.
The USS Enterprise-D receives an automated distress call from satellites orbiting a human colony on the planet Bringloid V, which is in danger from solar flares from its star.
The colony turns out to have been founded by the crew of the SS Mariposa, a DY-500 class cargo freighter launched from the Baikonur Cosmodrome, USSR, Earth on 2123-11-27 and destined to the Ficus Sector.
The colony, now 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. Flying to a star located half a light year away, the Enterprise-D also visits this colony, which has taken to calling itself Mariposa. A "descendant" of the original Walter Granger, Prime Minister Wilson Granger, is happy to see the Enterprise and welcomes them to visit.
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 from the ship crash to establish a stable gene pool. This caused the Mariposans to turn to cloning instead, strongly rejecting biological reproduction, and as a consequence, any sign of intimacy. For almost three centuries, every Mariposan has been a clone derived from one of the five original colonists, and now the colony is in danger of dying out because of replicative fading: genetic errors will soon cumulate into a terminally fatal stage, making all subsequent clones inviable.
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.
An alarmed Wilson 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.
Under urging from Picard, Pulaski and Riker, the members of both colonists agree to integrate the Bringloidi and Mariposa colonies. However, monogamous marriage will at first be temporarily suspended, to ensure fast development of a healthy, sustainable new generation. As the survivors of the crash of the Mariposa were mostly male, and therefore the cloned colonists are primarily the same gender, Dr. Pulaski suggests that each woman will have to have three husbands of different genetic makeup.