When the TARDIS malfunctions and the doors open in the time vortex, the travellers and their Ship shrink to the size of insects. When they arrive on Earth, they uncover a murderous plot involving a new insecticide - but how can they stop the crime in their miniaturised condition ? (Originally broadcast in three parts.)

Ian and Barbara are accidentally taken into Smithers' laboratory and, while the Doctor and Susan face a hazardous journey to reach them, they witness the deadly effects of DN6 first hand.

The Doctor and his friends face a race against time to expose Forrester's plans and return to normal size before Barbara succumbs to the effects of DN6.