A pair of Raxacoricofallapatorians offer Sarah Jane a special plant as a token of friendship.
While trying to stop a Slitheen plot to turn the Earth into a giant diamond, Sarah Jane, Luke, Clyde, and Rani get unexpected help from two brown-skinned Raxacoricofallapatorians named Leaf and Tree, members of the Blathereen family. They claim to have dedicated themselves to hunting down and capturing the Slitheen. After joining our heroes for dinner, the Blathereen give Sarah Jane a plant called Rakweed, which they say will help solve world hunger as it can thrive in the harshest conditions. After Mr. Smith finds the plant to be harmless, Sarah Jane accepts.
The plant mutates overnight and releases its now deadly spores, infecting Luke. Sarah Jane goes to the Blathereen's ship, where she learns that Leaf and Tree are in fact the Slitheen-Blathereen and see the Blathereen side of their family as worthless "do-gooders." The Slitheen-Blatereen are farmers who are addicted to Rakweed and hope to turn Earth into the site of a new Rakweed crop.
The Rakweed spreads all over London, including Clyde and Rani's school. They see its spores infect their classmates and teachers. When the school bell rings, the nearby Rakweed is destroyed. Realizing that the sound is the plant's weakness, they tell K-9, who Clyde had brought to school to cheat on a test, to increase the bell's volume and destroy all the Rakweed in the school. With Mr. Smith's help, they are able to mimic the bell's frequency all over London, destroying the rest of the Rakweed and curing Luke. Sarah Jane tells Mr. Smith to prepare to mimic the sound once more, but only in the attic.
Clyde, Rani, and K-9 return to 13 Bannerman Road, where Clyde confesses to using K-9 to cheat on a test. Leaf and Tree teleport into Sarah Jane's attic and threaten to kill everyone. Sarah Jane orders Mr. Smith to produce the noise one more time. This destroys the undigested Rakweed in Leaf and Tree's stomachs, which produces methane gas and causes them to explode, spraying slime all over Sarah Jane and the others. Our heroes celebrate with a barbecue.
- Alien Catnip: The Rakweed.
- Hilarious in Hindsight: The Slitheen plot to turn a planet into a giant diamond. I guess they were finally successful.
- Running Gag: The cast getting showered with slime.
- Sarah Jane burning food.
- Toilet Humor: The SB's are killed by a buildup of methane gas in their bodies. As Clyde puts it, they fart themselves to death.