Believing they were on route to Edwardian era Brighton,the doctor and Leela touch down in the TARDIS at the Fang Rock light house. Inside the keepers,led by former army sailor Reuben,are facing a mysterious occurrence. Another of the keepers Vince has witnessed a a strange light in the sky. This is followed by electrical losses at the light house,and a strange cold fog and finally the death of another of the keepers named Ben. Upon arriving the doctor and Leela,identifying themselves as foreign travelers are instant suspects. Until,that is a yacht crashes into Fang Rock and the keepers are forced to bring aboard the beleaguered passengers. These passengers,one Colonel Skinsale along with Lord Palmerdale,a lady named Adelaide and a man named Harker instantly begin accusing themselves upon arrival of the reason for who stranded their yacht at Fang Rock.
As Vince continues to keep the coal fires burning to keep the light house running,Reuban is compromised and (without anyone else being aware) first kills Palmerdale and than Harker. It is after all this the doctor concludes that an alien form of some variety,that is temperature sensitive,is behind this mayhem. Upon cornering the compromised Reuban the doctor discovers he is in fact a shape shifting alien called a Rutan,from a frozen planet who intend on using the Earth as a colony after being hunted by the Sontarans once,of course they sterilize the human population planet so they can inhabit it's then frozen oceans. The doctor,with the help of Skinsale,Leela and one of Palmerdale's diamonds,is able then to construct a mammoth light beam that effectively destroys the Rutan mothership and averts the threat to humanity before he and Leela depart in the TARDIS.
Aside from the threat of the form changing Rutan,this story is primarily a character piece focused on two sets of people: the main three keepers of the lighthouse and the crew of the marooned yacht that crashes at Fang Rock. As the doctor points out very well,though the educational back round of both sets of characters are in sharp contrast,both are driven by superstition and ,in Adelaide's case,primal fear over the alien threat they are facing. Leela,typically prepared to violently confront their enemy,actually ends up being the voice of reason for the squabbling human parties. She also manages to interject one of this stories few humorous moments when,at the doctors request she pulls a Freudian slip when she reminds Vince to "keep the boy pressure up",meaning to use the word "boiler. Overall an excellent Earthbound Doctor Who tale benefiting largely from mood and atmosphere.