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.
A young lad,Phillip comes to stay with his aunt for the holidays,and joins in with the barnstormers and their rehearsals for their plays.
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.
The group find their space rocket destroyed and vow to find the culprit, unaware that the whole future of the Old Mill is at risk.
Eddie agrees to help the group with the play and Ludgate instructs Camberley to do whatever is needed to obtain the silver casket.
Having discovered the casket is missing the group make a plan to regain it, still unaware of Ludgate's plans to demolish the Old Mill.
The group discover Philip's secret and Rusty is desperate to investigate the tunnel.
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.