Instructions
Start a new game from the main menu. This will launch a 72 mode game. The 72 mode takes place over 72 hours, during which you have to solve cases which will allow you to get clues about the causes of the zombie outbreak.
Finish the 72 mode with an "A" ranking. To obtain an "A" ranking, you must solve all cases before time runs out. You must also talk to Isabella at 10 a.m. on the last day and be on the helipad which is on top of the mall at 12 p.m. on the same day. To talk to Isabella, simply go to the supermarket. Then make sure that you have solved all of the cases before heading out to the helipad. After you have obtained an "A" ranking in this mode, you will gain access to a new mode, called the overtime mode.
Complete the game's overtime mode. You can access that new mode from the main menu of the game. The overtime mode is the last chapter of the game and it takes place on the third day, after your character reached the helipad. Finish this mode. You will then be able to access the infinity mode from the main menu.