Doctor Who: Terror of the Zygons (1975)
This story opened Tom Baker's second season as the eponimous time-traveller. This was a period of big change for the series (now the longest running science fiction TV programme in the world). New producer Philip Hinchcliffe (later to be an STV controller of drama) wanted to make the series more adult - appealing to big brothers and parents as well as the traditional children's audience. Consequently, the stories and plotting are more complex and there is less focus on man-in-a-rubber-suit monsters.
There's also a grit and quality to the production values that adds to the believability of the whole fantasy. Douglas Camfield, one of the show's best directors, captures the action in vivid style, conjuring up the setting of Loch Ness.
Answering the Brigadier's space/time telegraph, the Doctor (Tom Baker), Sarah (Elisabeth Sladen) and Harry (Ian Marter) arrive in the village of Tulloch, near Loch Ness. A series of attacks have taken place on local oil rigs, and many are blaming the legendary monster. The Doctor discovers the creature to be the Skarasen - cyborg pet of invading aliens the Zygons. Their own planet having been destroyed and their spaceship crippled, these deadly shape-shifters are now intent upon taking control of the planet Earth.
Although set around Loch Ness, the show was actually filmed in England but to the uintrained eye, it could very well be the home of Nessie.