The story is set within two time frames. In the present, you find yourself locked up in the courtroom. You're getting acquainted with the evidence, witness testimony and questions posed by the Judge. Each question triggers a flashback - and begins your testimony. During the flashback you find yourself in a Parisian district, with each of your decisions, interactions and action affecting Alfred Ethons' testimony and the final verdict.
To help you in your testimony, you'll have access to all the evidence gathered on the case - expert opinions, reports and witness testimonies. Each of them can be (but doesn't have to be!) refuted in a number of different ways.
In the Evidence menu you can familiarize yourself with testimonies of witnesses, forensic expertises, physical evidences, etc. They are grouped into appropriate categories, making them easy to find.