You arrive inside a cage. Note the linking books, each with a graphic symbol of a different island. Turn until you see a button on a star shaped design mounted on the bars. Touch it to call Gehn. Gehn then walks over and starts talking to you. He tells you that he is a changed man. It is up to you whether or not you trust him. He asks you to go through the trap book first so he can satisfy himself that it is indeed only a linking book to D'ni. When he holds the trap book in front of you, touch it. You are now inside the trap book. Watch Gehn decide to follow you. He does so and he gets trapped in the book whilst you are now free, OUTSIDE of the cage. Find the switch that turns on the power to the other linking books in the room and move it to the right. Find a lever next to a window, and pull it to lower the bars to your former cage. Find a tunnel leading down a ladder to Gehn's bedroom. Examine the various artefacts in the room. Go to the bedside table and examine his personal journal. Click on the grey metal sphere on the table. Listen and note down the sequence of sounds. Go back up the ladder to the main room. Go into the cage area. There's a book that links to Prison Island here, the one with a single small square as an identifying graphic. Link to Catherine's island.
