R is for... Rakweed

When Sarah Jane Smith, Luke, Rani and Clyde were menaced one day by Slitheen, they were rescued by a pair of Blathereen named Leef and Tree. These were a rival clan from Raxacoricofallapatorius, orange in colour, who despised the Slitheen.

Leef and Tree had come to Earth to offer a gift - a flowering plant known as Rakweed, which grew profusely on their home planet. It could be processed to create a highly nutritious foodstuff which could alleviate famine on the planet.

That night, the plant given to Sarah as a sample began to germinate, producing hundreds of spores. Breathing them in, Luke fell dangerously ill. The spores spread out through an open window, and soon the plants were beginning to sprout up everywhere.

Clyde had to sit a science test the next day, and decided to smuggle K-9 into the classroom in advance to help him. Rakweed had established themselves in the lab, and these began to emit more spores.

Sarah was able to use super-computer Mr Smith to work out what was going on. The parasitic Rakweed would soon destroy all life on Earth, as the spores were programmed to eliminate any lifeform which might threaten its existence. 

It transpired that Leef and Tree were actually cousins of the Slitheen, harbouring the same genocidal intent. It was discovered that the plant was susceptible to high frequency sound - noticed when the spores shrank from the school bell. K-9 and Mr Smith were able to replicate and broadcast this frequency across West London to destroy the Rakweed and save Luke.

Blathereen were addicted to the plant and, after gorging themselves on it, Sarah used the frequency to destroy what Leef and Tree had just eaten - blowing them up.

Appearances: SJA 3.6 The Gift.

• Leef and Tree were voiced by Miriam Margolyes and Simon Callow respectively.