Please note that this solution is for the PC version of the game. For the C64 version see this solution.
Playing as Dick:
Talk to Aunt Fanny about the professor and the wreck. Then walk to the left to enter the beach. Talk to Alf working at the boat. He doesn’t know where George is. Look at the footsteps in the sand and then use the box to push it aside, revealing more boxes. But you can now walk to the right, into a cave. There’s George, and Timmy.
Talk to George and ask her about the wreck, her hideout and the professor. On the chest is a LANTERN WITH CANDLE and in the sand is a LAMP. Take them both. Then walk to the right back to the beach to return to Aunt Fanny.
Playing as George:
At the kitchen table talk to everyone. Take the SANDWICH from your plate. Use the cupboard to the right of Aunt Fanny to take the OLIVE OIL out and use the teapot on the stove to pour your dad another cup of tea. As soon as he drinks it, give the sandwich to Timmy and your plate is empty. You’re now allowed to leave the table.
In the hallway, when you try to enter the study on the left it doesn’t go unnoticed because the door makes a terrible noise. Walk south to leave the house and walk to the shed on the left of the house. In front of the shed is a BUCKET. Before going into the shed, take it. Inside, take the SCREWDRIVER from the workbench and on the wall is a piece of ROPE.
Below the rope you can find an AXE and on the workbench there’s an empty OIL CAN as well. Leave the shed and go back into the house. In your inventory, use the olive oil with the empty oil can and then use the oil can on the door of the study. Now you can go in unnoticed and listen to the conversation.
The next morning at the kitchen table, you can’t leave the house until the map of the wreck has been returned. Talk to Julian and you’ll end up in the hall at the top floor in front of the guest room. Use the key hole in the door to see the key on the inside. Use the newspaper in your inventory on the crack under the door and use the pocket knife on the keyhole to poke the key out and gain entrance.
Look in the pocket of the coat hanging on the open wardrobe door to find a KEY inside. But where would the key fit? Back down in the hallway, use the key on the suitcase standing in the hallway. Inside you’ll find the map so you’ll have to tell your dad. Enter the study on the left and tell the whole story. When Anne enters the room, talk to her. Then talk to your dad again and you’ll end up in the kitchen.
Talk to Julian to come up with some ideas. Tell him you have to look in the guest room again and use the door to leave the kitchen, then use the door upstairs to enter the guest room. Pull the rug in front of the bed to reveal a BOOK lying on it. Take it, leave the room again and enter the kitchen.
On the beach, talk several times to Joshua, the fisherman. Ask him everything you can. Once he wishes you good luck with the search, use the rowboat to push it into the water and you’ll be on your way to the wreck. Go over to it to take a closer look.
Playing as Dick:
Open your inventory and take the rope. Use it to throw at the ships rail above you and you can climb on board. Inside the ship there’s a cupboard on the right but it’s locked. If you use your screwdriver on the hinges you can open the door anyway and take the TIN from it. Walk south to return to the other four. Suggest that you should open the tin first.
Sail south twice to return to the beach. Walk up the stairs to Kirrin Cottage and enter the house. Go into the kitchen and look at the note on the table. Next to the door is a tin opener. Use the tin from your backpack on the opener and find another KEY inside.
Use the door to leave the kitchen and the others will join you. Telling about the key, now you need to find a way to get into the study unnoticed.
Playing as George:
Enter the study and ask your dad to look around. He’ll reluctantly lets you but when you look at the model ship above the fireplace he’ll send you out. Enter the kitchen and take the SANDWICH from the worktop and take the TEAPOT from the stove. Leave the kitchen and enter the study.
Use the teapot on the cup on the desk, then use the teapot on the fireplace to extinguish the fire. Take the LOGS from the basket and put them in the fireplace. But now it’s getting pretty hot in here. Use the window to open it and then leave the room. Leave the house and outside you can now use the open window to get back into the study.
Use the key on the model ship and find a MAP inside. You’ll take it. Now leave the study through the door and you’ll meet your cousins in the cave. After the conversation, leave the cave and use the rowboat. Go to the right of the wreck to land on the island. Walk the path north to the castle ruins.
Once everyone is looking for an entrance use the bush to the left of the tower and Timmy will sniff around and fall into a hole. Now you need to get him out of there.
Playing as Dick:
Tell George you need to clear the bushes first. Use the axe from your inventory to make some room. When you use the bushes you can have a closer look but see that the ladder is unusable. Leave the hole and use the rope in your rucksack on the stone next to the hole to lower it and when you use the bushes again you can lower yourself into the well.
To get Timmy out of there, tie the bucket to the rope and your siblings can hoist Timmy up. You can climb back to the courtyard as well. Talk to Julian and you’ll tell him about the draft. Talk to Anne and she’ll prepare a picknick but finds a loose stone. Use the stone to try to move it and talk to Julian to get some help. You’ll reveal an entrance to the dungeons. Use it to have a closer look.
In the dungeon, walk to the left to enter a labyrinth. Walk south to reach a fork in the path. Walk south into the passage (the chamber on the right is empty). Now you can go three ways. Left to another chamber, right into the labyrinth or north into the labyrinth. Go north and you’ll find a door to the treasure chamber. While you use your axe on the door to open it, George will get the others.
You’ve certainly found the treasure! But so has the professor and his accomplice. You’ll be locked up in the room next door. Examine the door and find a small crack at the lower left. Timmy will dig a hole and once he’s on the other side you can open the door and enter the labyrinth again. Now you need to get out of the labyrinth.
Walk into the labyrinth tunnel on the right and you’ll find the wall with a hole in it that leads to the well. Enter the hole and you’ll end up on the beach. You’ll come up with a plan. Walk north to follow the path to the castle ruins. Take the ROPE from the well. Pick up a few STONES from the pile you’re standing next to. Take one from your backpack and throw it at the bushes next to the well. Once the professor has returned to his spot, repeat that to get rid of him… for now.
To make sure the professor doesn’t come out the other end, use your rope on the ivy on the wall on the left to let the rocks cover the entrance. Now you need to get some help. Use the radio to call Alf and ask for the police.