The giant lizard was an ordinary lizard subjected to Professor Tansbury's experiments. In a disused lighthouse off the coast of Porthferiot, Cornwall, Tansbury harnessed the power of a lightning storm to accelerate the lizard's growth. The circuits overloaded and the lizard grew to giant size, destroying the lighthouse and killing Tansbury. This was witnessed from shore by a local fisherman.
A few days later the Fourth Doctor and Sarah Jane Smith visited Porthferiot to investigate. After the giant lizard destroyed a fishing boat, the Doctor had UNIT call in the Royal Navy. Against the Doctor's advice, they tried using depth charges to kill the creature, but this only angered it. After it destroyed the Navy ship and went on a rampage, the Doctor used an amplified recording of the lizard's mating call to lure it into a harbour, where UNIT soldiers killed it with a missile. (COMIC: The Tansbury Experiment [+]TV Comic special comic stories (1977).)