The story features Tree and Leef Slitheen-Blathereen, descendants of an offshoot branch of the Slitheen family of Raxacoricofallapatorius that the protagonist Sarah Jane Smith has faced several times before in the series.
They gift Sarah Jane and the Earth a Rakweed plant that spreads spores to both quickly grow and wipe out humanity for profit.
Claiming to be law-abiding bounty hunters, Tree and Leef offer a plant called Rakweed as a gift over dinner with Sarah Jane, Luke, Clyde, and Rani.
They then realise that the Rakweed's weakness is noise of a certain frequency, which interferes with its communication signals and prevents it from growing, thus killing it instantly.
[1] Pearson Education published a simplified novelisation of this episode by Trevor Baxendale under the title Blathereen Dream for school literacy programs in September 2010.