On a planet filled with strange wild butterflies, the TARDIS travellers meet Professor Gnat, a zany butterfly collector. When the Doctor finds a dead cybermat, he knows the Cybermen are close but Gnat doesn't heed his warning and is captured and taken to a Cyber-City. The Doctor looks for more dead cybermats, deducing the flowers' odour is deadly to the Cybermen. Using the flowers they enter the city and free Gnat. Using a ray gun, the Doctor tunnels their way out, but causes a cave-in behind them. The Cybermen think they're dead. The travellers escape and head for the TARDIS while Cybermen fire at them and escape just as Gnat catches a butterfly for his collection.