“Luther: The Fallen Sun” (**1/2 out of four) was an initially taut adaptation of the BBC television series showing the disgraced detective John Luther (Idris Elba) in prison; when a vicious serial killer (Andy Serkis) begins terrorizing London, he realizes it is up to him to stop him and he breaks out of prison and evades the police and attempts to penetrate the London underworld to find him. Elba is commanding as usual in the title role and helps you overlook story flaws and contrivances but film loses its way before its improbable and silly conclusion. A good example of a television series that starts to show its cracks when expanded to feature-length film.