OK so basically my idea that in 2021 an English cruise ship docked of the coast of India suddenly gets pulled into some kind Primeval style hole in space in the water that deposits them in a lake on an alien planet. Before the ship sinks they all pile into the life rafts and get to sore where they eventually set up a small colony farming the giant flightless birds and pygmy elephants native to the planet (sea birds and Indian elephants being the only other animals to be pulled through to this planet on past portal openings).
Eventually after about 27 years on the planet the colonists undergo an ideological split where in about half of it's members want to keep the ship's old captain as their leader and the other about half want to democratically elect their leader. upon finding their differences unrecognisable the group decide to go their separate ways with the one with least members which turns out to be the dictatorship party, deciding to go and set up their own community else where.
1700 years later the two societies have grown into the two most powerful superpowers on the planet with which all the other dozen or so countries are ideologically and politically allied with one of. In terms of technology the planet has now reached a level similar to but clearly in advance of present earth level (think Battlestar Galactica) complete with large interplanetary but still sub-light space ships. It is in this period that the people of the colony Planet (now called Porta mundi) rediscover the location of earth relative to them.... and amazingly it's only 23 light years away. To avoid conflict the two super powers agree to build two identical ships that will travel together on the 30 year trek to earth where they will both be allowed to make their own attempts to establish positive relationships with the surly massively advanced people of 38th century Earth.
30 years later to their horror the crews of the twin ships find upon Arriving in earth orbit that the planet's inhabitance are not advanced at all. It turns out that in the mid 21st century a flu pandemic wiped out 99% of the earth's population and set technological development back 1000s of years, only just now in the 38th century have the people of earth reached a roughly AD1450 like level of development again. Having no choice but to stay the ships brake formation from each other and both make crash landing on different uninhabited parts of earth. Eventually the two crews come to the realisation that even if earth's not advanced it's still a reasonably sparsely populated habitable planet and thus still very important for them to each gain control of. To the aforementioned goal the two crews start sending out expeditions and troops to the different medieval nations of earth to try and win their allegiance for when more of their receptive peoples arrive.
That finally is what the show would focus on, the troubles and conflicts the two crews would run into as they attempted to gain control of the post-apocalyptic Earth.