Basically, Ray Milland's character wants to kill Grace Kelly because she's cheating on him. But planning the perfect murder isn't so easy after all...
The film was originally shot in 3D and there are some moments where 3D would have enhanced the thriller aspect but it's all a bit old-fashioned for me. If you love this type of plot-heavy mystery where everybody lingers over the details of a murder until the conclusion, you will love this. If not, you'll remain indifferent.