Five youngsters look for somewhere to rehearse a play,they shelter from the rain inside an old mill in a barn,and decide to call themselves the barnstormers.
The group continue to rehearse the play. Meanwhile, when out looking for some props, Rusty and Sarah hear more about the story of the Pirate Alexander Martins.
Philip takes an interest in the old casket and is sure that it is the same one as in the old painting. Meanwhile a stranger is seen watching the old mill.
Rusty explores the tunnel, and gets his foot stuck, there are rats everywhere, Mike and Phillip find him but water is leaking through. The alderman gets wind of the plan to demolish the mill.
Rusty, Phillip and Mike escape from the tunnel, they find a ladder which leads to the swan house. In the tunnel they find a chest containing a gun and a silver key. The key fits the casket and they find the treasure, a diamond necklace, inside.