The “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 the couple in a grim care home.

The plot is overcooked like burnt toast. The movie opens with guardian Rivera’s knucklehead goons burying an elderly man alive in the middle of the desert like mafiosi, and overall it’s suffused with a slightly naff pseudo-noir mood. Still, after watching men over 50 like Liam Neeson and Clint Eastwood kicking butt, this feels like a gamechanger. More please.

• The G is in UK and Irish cinemas from 21 June.