Together with the female spaceship Paurusheya, Sigurd Westall reaches Saturn's moon Japetus. There he finds a huge lunar station embedded in the ridge at the edge of the dark hemisphere. The station is also the destination of the strangers in the small spaceship he has been tracking. Sigurd first tries to find his two abducted colleagues on his own. But when he is very quickly discovered by the aliens, he is forced to fight back. In doing so, he receives help from a side that plunges him into a personal crisis during the further plot.