Hetty and the gang find crucial clues to the Dawn Star, although Flo and Ambrose are now watching them closely and Lady Grenford appears to know more about the diamond than she's letting on.

Lizzie discovers a secret tunnel with a clue, while the gang races Ambrose to find the jewel, and Hetty fears that Ashok is growing too friendly with Lady Grenford.

The gang realise they need to break through a wall to get to the jewel - but can they do it without being discovered? And what will Agnes do when she finds out Gideon's been lying to her?