Alterim Maré (slightly adjusted Latin meaning “other sea”) was created at the same time as Earth as a kind of “second chance” for drowned sailors and seamen; the merfolk, who were created with it, are tasked to rescue drowning people from the Atlantic Ocean and bring them through the Threshold - an undersea portal to world which is almost a mirror version of our own. It's slightly set back in time, as it's largely populated by people who would have crossed the Atlantic; it was at one time a Viking empire, before the Conquistadores arrived. It’s very fashionable in Alterim Maré to learn as much as possible about the “Overworld” - so when new men and women are rescued, they are often highly sought after as companions (or something less friendly) because of their knowledge of Overworld fashion and technology etc. Many attempts have been made to return to the Overworld, but as far as anyone knows, no one has made it back.

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At the time of Call Me Captain, society is beginning to look very similar to the 18th century. Captain is an "Overworlder", who was rescued from the sinking Titanic in 1912.