Disguised as a school headmaster named Mr. Finch, Lassar and his brethren took over Deffry Vale High School in London and manipulated its students so that they could solve the Skasis Paradigm, a sort of "universal theory" that, if solved, could allow the Krillitanes to essentially become gods and remake the universe as they saw fit. Lassar tried to tempt the Doctor into joining him, stating that he could use the power of the Paradigm to save countless civilisations, including the Time Lords. The Doctor rejected the Krillitane's offer and he and his companions destroyed the school's computer labs, ruining Lassar's plans.
Lassar and the other Krillitanes chased the Doctor and his friends through the school. The Doctor, Rose, Mickey and Sarah Jane Smith escaped through a kitchen window, leaving the robot dog K-9 situated close to a vat of volatile Krillitane oil which the Krillitanes had been using to control the school children. When the Krillitanes broke into the kitchen, K-9 shot the vat with his laser which saturated Lassar and his brothers. The result was a tremendous explosion that destroyed the school and killed all of the Krillitanes.