The TARDIS takes the Doctor and Sarah to the planet Zeta Minor on the edge of the universe. An unseen menace has wiped out an Morestran expedition and haunts the alien jungle.
The Master arrives on Earth in his TARDIS, camouflaging it as a horse box in a Circus. He immediately contacts the Nestenes and assists them in mounting a second invasion of Earth. The Doctor and his new assistant, Jo Grant, have to tackle the Autons, the Master and a large number of deadly daffodils!
The Doctor and Sarah are held responsible for the deaths on Zeta Minor, but the Doctor knows that the real culprit comes from the mysterious universe of antimatter.
In the universe of antimatter, the Doctor communes with the creature and makes a bargain.
The Doctor investigates Rossini's International Circus. The Master, meanwhile, has assumed control at the plastics factory and is manufacturing new lines of living plastics.
