The Gift - SJA 3.6

In which Sarah Jane Smith and her friends pursue a juvenile Slitheen - only to learn that they have been lured into a trap. In an old factory building they are captured by a number of adult Slitheen, who are about to activate a matter condensing device. The entire planet will be crushed, to create a vast diamond. The plan is foiled by the sudden arrival of two orange-skinned Slitheen, who teleport the others away to their spaceship. The new arrivals introduce themselves as members of the Blathereen family of Raxacoricofallapatorius - Leaf and Tree. They explain that the reputation of their planet was ruined when the Slitheen family took control, as they were only interested in profit and could never be trusted. The Blathereen managed to take back control and the Slitheen were expelled. Leaf and Tree now travel through space capturing Slitheen criminals. They claim that they wish to give Earth a gift, and they would like Sarah to be their ambassador in order that humans will accept it. The pair then invite themselves to Rani's home that evening for dinner, where they will explain the gift they offer. Sarah agrees, but does not entirely trust them.

That evening they teleport into Rani's home. The gift they have brought is a small plant, which they call Rakweed. It can grow anywhere, and is highly nutritious. It could eliminate famine on Earth. The Blathereen themselves love to eat it. Later, Sarah reminds her friends that they have a biology test in the morning - which Clyde has totally forgotten about. She asks Mr Smith to analyse the Rakweed, but he can find nothing suspicious about it. The following morning Luke goes to the attic to fetch his tie and the Rakweed suddenly produces spores. He breathes them in and rapidly falls ill. This alarms Sarah as he is never unwell, due to the way that he was created. She decides to keep him at home and has Mr Smith analyse him. More spores are emitted, but Mr Smith sucks them out of the room.
Clyde and Rani go to school as normal, and Rani is shocked to discover that Clyde has brought K9 with him - intending to keep him hidden and feed him the answers to the test. They notice that there is Rakweed growing on the window ledge. Their teacher inhales spores from it and collapses. Soon there is news of a mysterious illness breaking out all over West London. Mr Smith identifies the Rakweed spores as the cause. The plant is growing all over the area and spreading further afield. Luke will die when the amount of toxin in his body reaches 100% saturation.

Sarah has Mr Smith trace the Blathereen to their spaceship, which is parked in Antarctica. She has the computer teleport her to the ship, but he only has enough power for a one-way trip. She goes armed with a gun loaded with vinegar. The Blathereen reveal that they are actually of the Slitheen-Blathereen family, and just as deadly and duplicitous as their cousins. They succeed in capturing Sarah. She learns that Leaf and Tree are addicted to Rakweed and can't stop eating it. They intend to ensure that it covers the Earth so that they can harvest and sell it. At the school, Clyde and Rani discover that the Rakweed spores are neutralised by the ringing of the bell. This particular frequency of sound can destroy the plants. Sarah manages to escape and get back to the attic. Clyde contacts Mr Smith and has him broadcast the frequency throughout the stricken area, using mobile phones, radio, TV and even car alarms. The weed is destroyed, and the infected people begin to recover. Infuriated, Leaf and Tree teleport to the attic to kill Sarah. She gives them one last chance to leave but they refuse, so she has Mr Smith activate the frequency in the attic. The undigested Rakweed in the stomachs of the Blathereen causes the pair to explode - showering everyone in orange goo. K9 informs Sarah that Clyde took him to school for the exam, so he is forced to clean up the mess as recompense for trying to cheat.

The Gift was written by Rupert Laight, and was first broadcast on 19th and 20th November, 2009. It was the final story of the third series of The Sarah Jane Adventures. It sees the return of the Slitheen to the programme after a break for the second season, though the main villains here are actually a pair of their orange-skinned cousins.
The Slitheen plan to turn the planet into a huge diamond is reminiscent of the villain's plot in the animated Tenth Doctor adventure The Infinite Quest, but there the plan was to turn all carbon-based lifeforms into diamonds.
Tree is voiced by Simon Callow (who has played Charles Dickens on two occasions in Doctor Who), and Leaf is voiced by Miriam Margolyes. Inside the costume are regular monster performers Paul Kasey and Ruari Mears, with Jimmy Vee as the child Slitheen.
Things you might like to know:

• An early draft featured UNIT as well as Mickey Smith. Noel Clarke was unavailable, so this story element was removed.
• The Blathereen had been mentioned in the first story of Series 1 - Revenge of the Slitheen.
• There has been one story each season in which one or more of the characters has been covered in gunge from an exploding alien (it always happens to Clyde).