On a dark night–the 12th April 1920–as a storm brews, the lighthouse on Beacon Island suddenly goes dark and the SS Essex County flounders on nearby rocks. As the crew abandons ship, Rupert Garfield (a researcher of natural world mysteries and phenomena), John Nelson (a stout sailor) and Solomon Redwell (an agent of the FBI) make it onto a lifeboat...
The strong swell carries them toward the Beacon Island–the only landfall close enough to provide shelter before the storm hits! As they near the island, they run aground on a sandbar and discover the wreck of another ship–recently sunk if the remains of the superstructure are anything to go by. With some work, John manages to get the boat free and make it to Beacon Island.
As the trio stagger ashore–wet and tired–the wind picks up and a light rain begins to fall. Much of the island is in darkness and after finding odd tracks around the door to the lighthouse keeper's cottage the trio explore the woods growing over much of the island. There they make a grisly discovery–a man lies eviscerated amid the signs of a sudden, desperate fight.
Growing concerned that something is terribly wrong on Beacon Island the three shipwrecked men enter the cottage to discover more clues that something is not right. Signs of a struggle and a trio of old gold coins minted in some unknown place by unknown hands lie about the ground floor. The cottage seems abandoned.
As the wind picks up and a hard, merciless rain falls the three climb the lighthouse's spiral stair to its zenith. They find a broken radio and spare bulbs. They also find a horrible scene of violence and death. A man lies dead by the shattered lighthouse light. Two strange fishy frogmen lie dead nearby as does a horribly deformed human wearing a thick trench coat.
As they examine this horrible scene, suddenly the lights in the cottage go out. A splintering of wood downstairs heralds the approach of a veritable horde of horrible frog-things that hop and jump up the lighthouse stair toward the besieged men.
The men–desperate, trapped and terrified–hold the frog-things off with a hail of gunfire and burning oil. Amid the chaos Rupert throws a bag of old gold coins he had previously collected about the lighthouse at the things. The surviving frog-things pick up the coins and retreat leaving two of their number aflame on the stairs from where John had cast oil and a flare.
With the frog-men-things driven off the three had nothing to do but wait for rescue. Luckily, rescue was not long in coming, as the lighthouse's darkness, and the sounds of the 22 Essex County breaking up on the rocks, had been noticed from shore.
Finally back on the mainland the three men report to the local FBI office where they tell their tale, fully expecting to be derided as madmen. Curiously, the officials believe their story! Even more curiously, the FBI confiscate all their clothes and possessions and then swear the three to secrecy!
What did the FBI know? Why did they believe the survivors? And why did they burn the three men's clothes?