Young man (late teens to late 20's) gets transported to a new location/time by (insert own means: accident, aliens, caught by edge of experiment, genie, whatever..).
He arrives unconscious and is found and cared for by a young woman and her family. He has little to no memory when he wakes up and is very weak/injured. Takes a long while to heal and learn to speak their language during which time he falls in love with the young woman.
He wants to marry her but finds out that no marriages are allowed between a citizen and an outsider, so he would have to go to the capital city and petition to become a citizen.
He has slowly started to regain some of his memories during this period and recognizes that the society he is in is a curious mixture of primitive and advanced (food is combination of hunted meat and farmed grains while travel is done by vehicles which only require a citizen's rod to be inserted into an opening for it to move - citizen's rod is tool given to each citizen upon their reaching their majority, has many more uses that just powering their vehicles, other uses up to author to describe).
He decides to travel with her to the capital city, Ata' Lantais (City of Lantais), to petition for citizenship and during the journey starts to piece together some clues about the society (primitive but advanced, name of capital city, hints that other outsiders are much more primitive even brutish, geography of the small nation, etc) and slowly realizes that he might be in the mythical Atlantis...
Possible continuation to include being given a task to become a citizen such as helping to discover a way to isolate their nation from the brutish outsiders that are constantly trying to attack them across the small mountain range that separates their peninsula nation from the rest of the world. Possible solution could be taking a lot of the "citizen rods" and using their contained power to try and physically separate the peninsula from the mainland. Such an action could end up being the actual cause of the final calamity that destroyed Atlantis, making the MC's desire to earn his citizenship so he could marry his girl becoming the driving force to the destruction of their entire society.