Brian May described this song as his own little science fiction story.
It concerns a group who set out in a space ship, sailing ‘across the milky sea’ (which is a reference to both the Milky Way and to a pre-Queen band of Freddie Mercury’s, Sour Milk Sea), in search of a newer and better world. Their quest is ultimately successful, but their return to Earth That Was sees them fall foul of relativistic time dilation.
’39 – Queen
More details will of course be provided, but not until 2139. Or you could listen to the song, which is awesome…