A chillingly unexpected spin on the geriatric horror genre, James Ashcroft’s nervy New Zealand psychological thriller combines two credible threats to unsettling effect. One is dementia and the dread of a once sharp mind losing its edge and autonomy. The other is a demented senior psychopath with an eyeless doll repurposed as a glove puppet, who terrorises the fellow residents of his nursing home. The Rule of Jenny Pen plays out as a battle of wills between former judge Stefan Mortensen (Geoffrey Rush), newly arrived as a patient at the facility after a stroke, and the monstrous, cackling Dave Crealy (a thoroughly chilling John Lithgow) and his demonic rubber sidekick, Jenny Pen.

There are moments of greatness in this bracingly malicious tale of elder abuse. The opening sequence, which captures the disorientation and reality slippage resulting from Stefan’s stroke, is superb. The use of sound and music (the chirpy cockney anthem Knees Up Mother Brown takes on a sense of choking menace) is first-rate. And both central performances are impressive. Where the picture founders is in a repetitive screenplay that clumsily switches character perspectives and squanders much of the story’s initial promise.

• In UK and Irish cinemas