the Citadel back. When Kelner and the Vardans accuse the Doctor of being untrustworthy, he offers to show his commitment by dismantling the quantum forcefield that surrounds Gallifrey, which would allow the Vardans' full invasion force to arrive. However, the Doctor tricks them, and only creates a small hole in the forcefield, large enough for K9 to trace the Vardans to their home planet and engage the time lock, causing the Vardans to disappear. The situation seems resolved but moments later, a squad of Sontaran warriors transport into the Citadel. Their leader, Commander Stor, explains they had used the Vardans to help disable the forcefield, and seizes control of the Citadel. While Kelner quickly aligns with the Sontarans, the Doctor, Borusa, and their allies escape, regrouping with Leela, Rodan, Nesbin, and the other outsiders. Leela joins Andred to help defend the Citadel with the combined resistance group and Citadel guards, while Rodan repairs the hole in the forcefield. The Doctor wants the Great Key of Rassilon, but there is no record of it. He deduces that Rassilon gave the key to the first chancellor and that its location has been handed down from chancellor to chancellor ever since. He convinces Borusa to give it to him, which he plans to use to power a Demat Gun, a weapon that erases its target from all of time. Stor learns of this and orders a squad of Sontarans to give chase in the Doctor's TARDIS with assistance from Kelner. The Doctor is able to elude them within the labyrinthine corridors of the TARDIS, and with Rodan and K9's help, constructs the Demat Gun. He goes out into the Citadel to find Stor, who is attempting to destroy the entire galaxy, including Stor and the Sontaran fleet, with a bomb. The Doctor fires the Demat Gun, and the Sontaran invaders disappear, ending the threat. When he awakes, the Doctor remembers nothing of the events, and Borusa calls it the "wisdom of Rassilon", as he can return the key to its hiding place. He then has Kelner arrested for treason and starts the process of rebuilding the Citadel. As the Doctor prepares to leave, Leela announces she plans to stay behind with Andred, as they have found a romantic interest in each other, and K9 also insists he must stay to help Leela. After saying his goodbyes, the Doctor enters the TARDIS, pulls out