Doctor Who and the Power of Kroll — Terrance Dicks

The Doctor and Romana have arrived on the third moon of Delta Magna, searching for the penultimate segment of The Key to Time, finding themselves caught in the middle of a dispute between the crew of a Methane Refinery and the natives (known as 'Swampies'). The Swampies claim the crew have disturbed the waters, and will incur the wrath of Kroll. It turns out Kroll is a giant squid which surfaces every few centuries. Once of normal size, it ingested the fifth segment of the Key to Time and began to grow and became a god-like figure to the Swampies and their descendants.
After Kroll awakens and begins to attack both the Swampies and the Refinery indiscriminately, the Doctor uses the tracer to eliminate Kroll and retrieve the segment of the key, saving the planet's inhabitants in the process. Both he and Romana go back to the TARDIS and set off on their next adventure.