In Criminal Case, the player acts as a detective to solve murder cases, locating relevant clues at crime scenes by clicking items in the scene. Rapid clue location is rewarded with greater scores; the player earns stars that can later be used as a type of currency for performing necessary tasks such as examining evidence and interrogating suspects. At the conclusion of every case, the player is presented with all the suspects and must arrest the one who fulfills each of the criteria gained throughout the investigation. If the decision is correct, the case is solved. To unlock the next case, however, the player is also required to complete the solved case's post-indictment chapter, enabling the player to receive reports and finally move on to the succeeding case—though this procedure does not have to be undertaken in any of the tutorialcases. There are also puzzle elements and various progression and bonus features integrated into the game.
The team first encounter Diego as a suspect in a murder during a lighthouse soirée. Due to his shady background and character, the Squad track him to a séance, where they discover him to be a forger. Later, he forges a ticket to Alastor's next party for the Squad in order to get released. He becomes a suspect once more before proceeding to assist the Squad (especially Evie) in clueanalyses on numerous occasions.
Later, a shootout between the Italian gang and the Squad leaves Evie injured, throwing Diego into a panic. After they are able to get her help, Diego acknowledges that he wants to be part of the Squad and thus officially files a request to join, getting instated in the team shortly thereafter.