Jeff and Andy Stone are still looking for their sister Mary Stone when all the Daleks on Venus vanish. They're high atop Winston Dam with the newly-appointed commander of the Space Army, Vel Karneen, when something odd strikes them about the Winston Mountains. The middle of its three peaks is distinctly taller than the two outer ones. The brothers immediately assume that the Daleks must have added to the mountain. Surmising that Mary is probably inside this new Dalek base, they ask Karneen for the use of a copterjet and set off to investigate the mysterious addition to the mountain.