The King and the Castle — Peter Locke

It's the weekend, and Mr. Watson has arranged for Mikkee the Martian and the twins to visit an ancient castle. Billy is not amused. On arriving, they are escorted by a pompous guide through a tour of the castle. Mikkee sees the ghost of the king leaning against the entrance. He gives the twins the power to see him too. The king acts the clown and makes fun of the guide. The twins find this all very funny and are told off by their father, who, of course, cannot see the ghost. The king shows Mikkee a secret room upstairs and a secret passage in the kitchen. The three follow this to a smugglers' cave, and Jilly finds some treasure.