The Doctor (Jon Pertwee) and Jo (Katy Manning) take the TARDIS on a test flight. They arrive on a cargo ship, the SS Bernice, which seems to be crossing the Indian Ocean in 1926. In fact, the ship and its crew have been miniaturised and trapped inside a microscope - a banned entertainment peepshow - on the planet Inter Minor. After moving through to another section and being surprised by ferocious Drashigs, the Doctor eventually breaks out of the scope and reverts to full size. The Doctor's efforts to rescue Jo, still trapped in the scope, makes for a thrilling episode.