Sto-cats were huge stone robots. "Thousands of millions" of Sto-cats were created by a warlike race called the Kryptolians, and positioned on as many life-supporting planets. The Kryptolians' intended to awaken them (using a short radio-active blast as a signal) after "one million Brelian years".
One Sto-cat was mistakenly activated by the proximity of the nuclear thrusters of the Guerner Transport Ships as the ships landed on the empty planet where it was posted. The Fourth Doctor realised what had happened and managed to talk the Sto-cat into shutting itself down again, lest it endanger the plans of its creators.