A good mystery, especially one that rides on a big reveal, should feel like the audience had some chance of putting it all together. It is akin to watching someone solve a puzzle and thinking, "I could have figured that out." The third act of The Good Liar unfolds so many random pieces of the puzzle at the audience that no one could have possibly solved it. It is a ridiculous twist that doesn't add up. However, there are joys to be had before it that make The Good Liar still worth seeing.
The reason The Good Liar works well up until then is its stars. Helen Mirren and Ian McKellen have a blast playing off each other. McKellen as Roy is wonderfully hammy as a con artist who tries to con a woman out of her money. Mirren as Betty is able to capture the naiveté of a mark while always making us think she has the upper hand on him. Their opening date scene crackles with energy and wit. Roy thinks he has stumbled upon a great find in Betty. She is wealthy, has a heart condition and seems to trust him. Betty is something else entirely. Both have pasts that are hinted at through small details. Roy hates kitchens for example.
Roy runs around on the side, running more than one con at a time. He is a sinister character with bursts of violence erupting whenever he feels threatened. Betty's grandson (Russell Tovey) is suspicious of Roy from the get-go. He causes Roy to recalculate often. This interplay offers up some enjoyable turns as a result.
Bill Condon is a competent director. There is anything particularly striking about the film visually but the real pull here is the lead actors. The ending may feel tonally at odds with the frothy fun of the film's first two acts but there is enough here to enjoy if you like these two actors. The Good Liar doesn't know how to handle the tonal shift that the twist brings, taking the film from a cat and mouse thriller into something more about trauma and abuse. The audience is left on the side, never having a chance to have put it all together.