Lionel's an awkward autistic man with a side order of Tourettes.
Frank relies on Lionel's perfect recall as a kind of human surveillance system. When Frank's latest case gets violent and he ends up gunned down Lionel starts to pick at the thread and find out what's going on despite the misgivings of the rest of the team.
This is classic film noir, something you don't get a great deal of nowadays. There's corrupt city officials, heavies in homburgs, dubious slum landlords, classic US cars, a hidden maguffin (which I spotted instantly, I bet you will too), hot Jazz and a pretty woman in trouble.
The autism=savant superpower thing is as tiresome as it always is but not overused and having interacted with a couple of people with Tourettes I was unconvinced by Norton's take but at least it didn't just disappear any time it was inconvenient.
The mystery is not overly complex but takes a chunk of time to unpick with dead ends and drifting that feels more authentic than some super organised investigation. It's not even really resolved in that neat way most movies aim for with the bad guys all dealt with but again that feels more real.
What it suffers most from is excess length, you've got to be in for a long movie. 7/10 mostly because it's slightly off the beaten path.