In an alternate Victorian England, young Arthur and his sister Myrtle, residents of Larklight, a floating house in one of Her Majesty's outer space territories, uncover a spidery plot to destroy the solar system.

Reports of a strange phenomenon at the fringe of the galaxy and its connection to one of Father's old friends send the entire Mumby family, accompanied by Jack and other friends, to a far-off planet where they must find a way to prevent a new invasion of the solar system by giant moths.