You begin as Claire, waking up in a cell. After the initial cutscene, follow Moira. Your first objective is to escape the cell block. Head through the open door and proceed down the corridor. You'll encounter your first Afflicted. Use Claire's pistol to aim for headshots. Remember to switch to Moira (Triangle/Y) to use her flashlight to stun enemies, making them vulnerable to Claire's attacks. Moira can also use her crowbar for melee finishers on downed enemies, conserving Claire's ammo.
Continue through the prison, collecting any ammunition and green herbs you find. You'll soon reach a locked gate requiring a keycard. Explore the surrounding cells and offices. One cell will contain a Prison Keycard. Use it to open the gate. As you progress, you'll encounter more Afflicted, some of whom may be playing dead. Always be cautious and check bodies. You'll eventually reach a large open area with a central tower. Your goal is to activate the power. Look for a generator and a fuse box. Moira's flashlight can reveal hidden items, so use it frequently to find extra ammo or crafting components. After restoring power, proceed to the next area, which will lead you outside the prison walls.