A spaceship, perhaps. A strong electrical field. Against regulations. DOCTOR: The Beast of Fang Rock. HARKER: They're dead because of you. Just a minute. LEELA: Go up and tell the Doctor to come down, and do not let the others know. This lighthouse is under attack, and by morning we might all be dead. You can't possibly blame the lighthouse people. You're an employee.
Plan: A: inexplicable electrical problems; Q: What is keeping the lighthouse from maintaining a steady beam? A: a foolhardy ship; Q: What crashed against the island rocks? A: Four shaken and discontent survivors; Q: How many people join the people inside the lighthouse? A: an attack; Q: What does the Doctor think the people are being watched for is a prelude to? Summary: With inexplicable electrical problems keeping the lighthouse from maintaining a steady beam, a foolhardy ship crashes against the island rocks. Four shaken and discontent survivors join the people inside where the Doctor surmises they're all being carefully scrutinized, a prelude to an attack.