Madam Delphi was a Mandragora who wanted to take over the human race in order to save the Mandragora from dying forever. She also wanted revenge on the Doctor, and would use her power over Earth to gain that revenge.
Madam Delphi had the power to take over many people, including Wilfred Mott's friend Netty. The Tenth Doctor, Wilf and Donna began to battle Madam Delphi, who was about to launch her plan to destroy Earth. This was foiled by the Doctor, who used Netty's mind to trick Madam Delphi, sending her and the other Mandragoras back into space. (PROSE: Beautiful Chaos)