Struggling actor Foster Twelvetrees is invited to a large country home by Stewart Henderson to perform a dramatic reading for his family.

Outwardly, Stewart is complimentary and enthusiastic, but his more sinister intentions were made clear when earlier he secretly sliced a poster of Twelvetrees.

Whilst they chat, Stewart's sister Jessica and their Indian servant Patel begin searching through Twelvetrees' luggage.

The next day, after being introduced to a snake house underground, Twelvetrees secretly goes upstairs to see Stewart's mother: though kept behind a locked door she initially seems extremely polite.

Suspicious that Stewart is trying to change Victor's will to his favour, Reggie and Ernest resolve to stay and make sure that doesn't happen.

In the meantime, Verity persuades Twelvetrees to check up on Victor, and to their shock discover the bed in his room is filled by a dummy.

Petrified, Twelvetrees makes a hasty exit only to be pursued by Verity: she convinces him to come back after she reveals the true identity of his father and his place in his will: he is in line to take over his money, the house and its estates.

Going back to the house, Foster meets up with Verity again to find Jessica – in possession of his framed motto – and Agnes dead by the snakehouse.

The film ends with Stewart, Verity and the Henderson mother being taken away in a police cart, whilst a camera shot moves away from Foster beginning to dig in the large grounds outside the house to find the diamonds.

"[7] The Monthly Film Bulletin wrote: "The House in Nightmare Park shows every sign of having been put together with intelligence and polish.

The film is elegantly photographed, and the minor comic characters are all effective within certain limits; but despite the care that has evidently gone into the production, it never successfully resolves the central discrepancy between Frankie Howerd's vaudevillian technique and the Gothic setting in which he is placed.

The two elements co-exist uneasily for the first twenty minutes and then begin to clash, most noticeably at the climax, where one is frequently being asked to accept Howerd as an amiable hero figure, with the result that the action simply degenerates into farce. ...