A unique Doctor Who tale, in that it takes place entirely within the confines of the TARDIS. It's also rather quirky as we witness the main characters descend into a frustrated mania.
After scheduling the first two stories, an epic adventure was planned in which the TARDIS and crew would spend the following weeks travelling with Marco Polo to ancient China. This lavish production would be costly though, so The Edge of Destruction (aka "Inside the Spaceship") was devised to offset costs and provide some time to design the larger production.