T he “G” in this low budget revenge thriller is 72-year-old Ann Hunter. That’s “G” as in “Granny”, a nickname given to her by her granddaughter. The joke is that Ann does not fit anyone’s stereotype of a granny.

To call her hard-as-nails would be an understatement: nails can bend, but the G is harder, cold, mean and sarcastic. When she reaches for a bottle, dirty washing stacked up around the sink, no clean glass to hand, she pours her vodka neat into an empty yoghurt pot. The G is played by character actor Dale Dickey, whose TV credits include Breaking Bad and My Name Is Earl.

On film, she’s best known for a supporting part in Winter’s Bone , but I have to confess I didn’t recognise her. Having seen her in action here, I’m a convert. The G is heartless and unlikable, her battered face set in a permanent scowl – and yet Dickey conveys a woman shaped by her experience; there’s a backstory eventually about her childhood in Texas explaining the rage and anger.

Her performance is a much-needed plus in a film that would otherwise lack a bit of substance. The plot is inspired by the real-life scandal of exploitation of older people in the US’s legal guardian system. It begins when the G and her sick husband become victims of a crooked guardian called Rivera (Bruce Ramsay).

He’s in cahoots with a doctor who declares they lack the capacity to manage their affairs. Rivera is appointed their legal guardian, sells their house from under them and imprisons.