After being attacked by the Rani, the TARDIS crash-lands on the planet Lakertya. On the floor of the console room, the Doctor begins his sixth regeneration... In his post-regenerative confusion the Doctor is separated from his young companion Mel and tricked into assisting the Rani in her megalomaniac scheme to construct a giant time manipulator. Lost on the barren surface of the planet, Mel has to avoid the Rani's ingenious traps and her monstrous, bat-like servants. She joins forces with a rebel faction among the Lakertyans, desperate to end the Rani's control of their planet.