Find the hidden safe behind the painting. Use the code 1135 (on PC) or 123 (other platforms) to unlock it.
Solve the puzzle involving the cash register and cat treats to get the next key.
Locate the colored gears to unlock the basement. Use a crowbar to break display cases and find the red gear.
Sneak into the museum to find clues. Watch for the Neighbor; hide in wardrobes to escape him.