The genetic splicer was created by Silurian scientists. It could combine the genetic material from different lifeforms, creating hybrids. The effect only lasted as long as the splicer was functioning.
Lady Cornelia Basildon-Stone found the only surviving Silurian scientist who had worked on the splicer while she was on an expedition in South Africa. She tortured the scientist on how it worked, then killed him.
The following year, Lady Basildon-Stone began implementing her plan of mutating herself into a part human, part wasp hybrid lifeform, then converting London's police into her obedient "drones" and becoming the new leader of the British Empire.
She did this by paying for a rainforest exhibition inside the Crystal Palace on Sydenham Hill in London. She had laid eggs in the exhibition's forest, and had all the glass in the Palace's roof replaced with glass that would turn the Palace into a greenhouse for the eggs to thrive in, and planned to make the Palace her own palace to rule from. She also had her servant Ajeeth use gas piped from the genetic splicer to convert the police into human-wasp "drones" that would single-mindedly obey Lady Basildon-Stone.
Later, Madame Vastra confronted Lady Basildon-Stone. During their fight, Lady Basildon-Stone took on wasp form and gained the upper hand. Meanwhile, the Sontaran Strax, who had snuck into the Crystal Palace at the same time as Vastra's investigations, threw a grenade into the genetic splicer. This caused Lady Basildon-Stone to become human while she was still holding Vastra in the air. Vastra got to safety, while Lady Basildon-Stone fell to her death.