At the local inn, the Inebriated Gremlin, they are served by landlord Piggy Warren, who introduces them to Mark Brandon, the school inspector, and Jill Manson, a teacher.

The following morning Steed and Mrs Peel examine strange footprints leading out of the sea and up the beach and find Smallwood dead and buried in the sand to their dismay and confusion.

Wondering where have all the people gone, they surmise that this is a gradual invasion by a foreign power: small groups have been dispatched from a submarine located in the North Sea, explaining the bootprints at the beach and adult sized Wellington boots at the school, replacing the locals one by one until only the invaders remain.

Steed and Mrs Peel are eventually found by some of the impostors; a fight ensues before they overpower them and leave after sealing the invaders permanently underground.

[2] The episode was well-received; most striking in the oppressive and poignant atmosphere created by the derelict school and airfield abandoned since the war and the sheer decadence of the village depicted in it.

In their book Reading Between Designs: Visual Imagery and the Generation of Meaning in The Avengers, The Prisoner, and Doctor Who, Piers D. Britton and Simon J. Barker stated: Inebriated Gremlin, the village inn where Steed and Mrs. Peel stay, are manifestly created for emotional impact: as an environment, the pub is palpably hostile and almost viscerally nauseating.

The lounge bar is thoroughly dilapidated and heavily cobwebbed, with a transistor radio perched on an old sofa cushion in lieu of the jukebox.

Chipped enamel basin, tarnished metal bedstead, tattered curtains and towels, a lumpy, ill-made bed, and a flypaper choked with its victims all proclaim the fact that visitors are not welcome.

Pottle manipulated the very space of the set so as to ensure that it is unsettling: the strongly inclined coving of the ceiling creates an oppressive atmosphere, which resonates with Steed's discovery that the windows are boarded up on the outside.