Julius Ricardo hitches a lift home across the English Channel from Lézardrieux to Dartmouth on a friend's ketch in response to an unexpected visit to London from his friend Inspector Hanaud. En route they pick up an escapee from a prison ship, who holds a grudge against Daniel Horbury, M.P. When Horbury is found dead at his home in Lordship Lane, Hanaud and Ricardo assist Scotland Yard in the investigation, which also involves Septimus Crottle, the patriarchal owner of the Dagger shipping line, and his extended family.