The Doctor and Mel join Alisha Hammerson, the Auton head of Hammerson Plastic PLC, at her Millennium Party. Their arrival is not unexpected and the Doctor is not surprised. He has been waiting to discover Alisha's plans. Alisha, having begun to replicate the businessmen in attendance at the party, reintroduces the Doctor to the Nestene Consciousness. A plastic meteor is the only weak link with the Nestene home world and even though the Doctor does not manage to destroy it with a liquid in a glass phial, Mel has a spare and uses it. The meteor is destroyed and contact with the Nestene is broken. The Autons, Alisha included, are rendered lifeless.
The Nestene, or Nestenes, create their form on Earth out of raw plastic, using a genetic code contained in their energy unit. It's their link to the nearest Nestene world, millions of light-years away in Tau. It can withstand the gravitic pressure of a black star.
"Millennium" is incorrectly spelt "millenium" on two occasions, including the title.
The idea that the Nestene can be destroyed with a liquid made by the Doctor in a test tube was later used in the television story Rose.