Twenty years ago, an expedition made up of representatives from a number of worlds disappeared on the planetoid Etra Prime, including the newly inaugurated President of Gallifrey, one Lady Romana. Some time later, many of their dead bodies materialised on the planet Archetryx.

Today, Archetryx hosts a delagation from the many planets who have time travel capabilities, including the Monan Host and representatives from Gallifrey. However, as the Doctor finds himself drawn to Archetryx, it becomes evident Etra Prime is being used as a weapon to destroy Archetryx. But why? What is the secret of the Gravity Wells and what is the significance of the crystals adorning the mutants that exist down there?

And just why does the incumbant President of the High Council of the Time Lords allow a Dalek Task Force to land on Gallifrey?