The Gift

Part 1
The story begins with Sarah Jane and her friends chasing an overweight boy. After chasing him into a warehouse, he is revealed to be a Slitheen working with another family member, in an attempt to use a matter compressor to compress Earth into a diamond, in order to 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 equipment, and Sarah Jane complies. Two orange Raxacoricofallapatorians appear and teleport the Slitheen away, stating that they have been sent back to their ship for trial. They then introduce themselves as the Blathereen, a separate, and according to them, law-abiding family of Raxacoricofallapatorians on a mission to track down members of the Slitheen and sentence them to death.

As a token of thanks, the Blathereen visit Sarah Jane in Rani's house for a meal of shepherd's pie, which the Blathereen enjoy very much. After the meal, the Blathereen—Leef and Tree—give Sarah Jane a Raxacoricofallapatorian plant called Rakweed. They explain that the plant "can grow anywhere, even in the harshest conditions," adding that because the people of Earth would be suspicious if the plant came directly from the Blathereen, that they would like Sarah Jane to be their ambassador. Mr Smith analyses the plant and deems it not to be dangerous, but the following night it starts to mutate.

The next morning, the Rakweed plant releases spores, most of which fly out of the window, though some are inhaled by Luke. Clyde and Rani go to school, and Clyde surreptitiously takes K-9 with him, intending to cheat on his biology exam. Luke then finds himself falling ill, which is unusual for him as he has never been sick before, so he stays home. Mr Smith analyses Luke, determining that the inhaled spores would proceed to drain him of energy, resulting in a coma from which he would never recover.

Meanwhile, Clyde and Rani are in science class preparing for the exam. Clyde uses an earbud device to enable K-9 to discreetly give him the answers to the test's questions, despite K-9's reluctance to do so. Just as the test begins, Rani spots a Rakweed plant growing in a nearby plant pot, which gains their teacher's attention. Just as the teacher inspects it, the Rakweed blasts a large amount of spores in her face, causing her to fall unconscious due to the illness. Everyone flees the classroom in panic and soon, the entire area is covered in Rakweed, resulting in several civilians affected by its spores to be rushed to the emergency room.

In the attic, Sarah Jane is staying close to Luke, whose condition has worsened and is now covered in a strange rash. Things then take a turn for the worse when the Rakweed releases more spores and the episode concludes with the spores hurtling towards Sarah Jane and Luke.

Part 2
As the Rakweed spores head toward Luke and Sarah Jane; Mr Smith reveals a powerful extractor fan behind his screen and removes the spores from the attic. Rani and Clyde attempt escape from the school where the Rakweed is taking over, but it becomes harder when they find Rakweed growing indoors as well as outdoors. Sarah Jane follows the transporter trace of the Blathereen to Antarctica, where she threatens them with a water gun loaded with vinegar. The Blathereen then apologize and announce that they will remove the Rakweed crop, claiming they never meant for it to go out of control. Unfortunately, this is revealed to be a trick and Sarah Jane is captured by the Blathereen, who reveal that they are from a mixed marriage of Slitheen-Blathereen. She also learns that they are addicted to the Rakweed, on which they are gorging themselves and intend to use the Earth to continue farming it.

Meanwhile, Rani, Clyde and K-9 are still attempting to escape from the Rakweed and its spores. Just when they find a window big enough for them to squeeze through, a nearby Rakweed plant unleashes its spores, but just before it strikes Rani and Clyde, the school bell starts ringing, destroying the Rakweed. They then realize that the Rakweed's weakness is loud noise, which disrupts its communication signals and prevents it from growing, thus killing it instantly.

After escaping from the Blathereen, Sarah Jane returns to find Luke on the brink of unconsciousness. With no time to lose, Sarah Jane makes contact with Clyde, Rani and K-9 and after explaining the Rakweed's weakness, they decide to use Mr Smith and K-9 to activate all sound-producing devices in the area at the loudest possible. K-9 gives Mr Smith the frequency for the school bell and within seconds, all bells, alarms, radios and car alarms go off and the loudness ultimately destroys all the Rakweed.

Luke is immediately cured and Sarah Jane is relieved to see him well again. Rani and Clyde return with K-9 and they come clean about Clyde's attempts to use K-9 for cheating on the biology exam. Sarah Jane tells Clyde that he does not need to cheat and needs to have more faith in his own abilities. Enraged at their plans being ruined, the Blathereen teleport to the attic and threaten to destroy everyone. Mr Smith generates the same frequency, destroying the partially digested Rakweed in their stomachs. The resultant build-up of methane gas in the Blathereens' stomachs causes them to explode. Clyde is told to clean up the mess and they all later celebrate with a barbecue.