Jack and Art visit Jupiter's moon Io, descending into Jupiter's atmosphere to ask aid of the Thunderhead, who tells them to protect the key to Larklight. Not knowing what this is, they attempt to leave Jupiter, but are abandoned by their ferryman and escape to a broken-down harpoon ship attached to a native organism. They are rescued by the "Sophronia's" crew. Jack discovers that Myrtle's locket (now in Art's possession) is the key to Larklight, in that it can activate a set of complex engines capable of transforming the solar system, and leads his crew to the spiders' home on the Rings of Saturn to exchange it for Myrtle's safe return.