The Blathereen, arch-rivals of the family Slitheen arrive on Earth. In a move that surprises the gang they declare peace between Earth and Raxicoricofallapatorius and they offer Sarah Jane a gift, one that might change the future of the human race forever.

Sarah Jane faces the battle of her life to save her son from the effects of the gift of the Blathereen. Meanwhile trapped in the school, Rani, Clyde and K-9 fight to find a way to get to the attic, where the final showdown will take place.