When the mudlarking gang find a mysterious cutlass on the banks of the Thames, they immediately know that something fishy is afoot. When the cutlass vanishes, and Mrs Drummonds disappears too, it's down to Clem and the gang to find them. The mystery leads them to a band of ghost pirates, who will stop at nothing to find their long-lost treasure. With the help from their friends in the Undercity, the gang must set sail on the seven seas, solve riddles and save the day!