Doctor Who and the Sontaran Experiment

Landing on Earth, now a barren, desolate planet, Sarah, Harry and the Doctor are unaware of the large, watching robot. The robot is the work of Styre, a Sontaran warrior, who uses all humans landing here for his experimental programmes.

What has happened to the other space explorers who have come here? Why is the Sontaran scout so interested in Earth and in brutally torturing humans, including Sarah Jane? Will the Doctor be able to prevent an invasion and certain disaster, and save both Earth and his companions?