After being selected as head of lower school, Darrell soon feels the demands of leadership when Felicity, her younger sister just starting at Malory Towers, gets into trouble.
Preparing for the inter-tower lacrosse match leads to disappointment for Darrell and Felicity. The rest of the team speculates why breakout star Clarissa is hesitant to play.
When Clarissa receives troubling news that her pony has been sold, she breaks the rules to rescue him. Irene and Mademoiselle Rougier make efforts to strengthen their relationship.
Darrell and her classmates await their exam while Felicity and June continue to explore the secret room. Gwen pretends to have heart issues to get out of the exam, but then finds herself in a dangerous situation.
Ruth's bullying of Connie reaches a new low when June is wrongly accused of playing pranks. Darrell encourages the lower form to find who is responsible and the whole class gets a lesson on accountability.
June and Darrell find themselves in danger when they leave the school to continue the search for the treasure in a cave during high tide. The rest of the school prepares for the wedding. Irene is given an important task by her father.