The story begins with Sarah Jane and her friends chasing an overweight boy. After chasing him into a warehouse, it is revealed that he was a Slitheen working with another family member in an attempt to use a matter compressor to squish Earth into a diamond and make themselves rich. After Sarah Jane's Sonic Lipstick fails, she calls K-9 who tries to destroy it. Annoyed, the Slitheen grab Rani and hold her hostage, threatening to kill her if K-9 continues to destroy their work. Sarah Jane complies. Two brown Raxacoricofallapatorians appear and teleport the Slitheen back to their ship for trial. They then introduce themselves as the Blathereen, a separate, and according to them, law-abiding family of Raxacoricofallapatorians who were on a mission to track down and sentence all members of the Slitheen family to death.
As a token of thanks, the Blathereen visit Sarah Jane in Rani's house for a meal of Shepherd's Pie (the Blathereen don't eat nicely at all). After the meals are out of the way, they (the Blathereen, now to be referred to as "Leef" and "Tree") give Sarah Jane a special plant called Rakweed. A plant that "can grow anywhere, even in the harshest conditions". The Blathereen explain that Earth would be untrusting if the plant was given straight to them by the Blathereen, and so they ask Sarah Jane to be their ambassador.
After Mr Smith had deemed the plant to not be dangerous, the following night it starts to mutate. The next morning, it releases spores which fly out of the window, but Luke, who was in the room, breathes them in. After telling Sarah Jane that he doesn't feel well, Mr Smith analyses him. The spores have infected Luke, and proceed to drain him of energy until he will fall into a coma which he will never recover.
Part 1 concludes with Clyde and Rani getting concerned that they see Rakweed in their biology lab, and Luke about to fall into a coma.
The spores come speading into the attic, and head towards Luke; Sarah Jane protects him, and Mr Smith brings out a fan behind his screen and gets rid of the spores. Rani and Clyde escape from the school where the Rakweed is taking over, finding out that the frequency of the school bell kills the Rakweed. Sarah Jane follows the transporter trace of the Blathereen to Antarctica where they are shown to be addicted to Rakweed.
After escaping from the Blathereen, Sarah Jane gets Mr Smith to re-program all electrical equipment in London to the same frequency and this wipes out the Rakweed.
The Blathereen teleport to the attic and Mr Smith produces the same frequency to destroy the Rakweed in their stomachs,killing them, leaving a mess in the attic for Clyde to clear up.