The pirates kidnap the courting couple and stow them away on their submarine, where they look out the porthole at sealife. Meanwhile, lass's father has reported the kidnapping to authorities, and offered them photographic plates in evidence. The navy to the rescue in their own submarine!! But wait! They can also fly!
Walter Booth was one of the early film makers who came from the background of a stage magician, and he and Georges Melies often covered the same subjects. Later on, he moved into story films with elaborate special effects, and this is one of them, using the techniques that had once formed the whole of his movies. While this one looks pretty crude to the modern eye, bored as we are with CGI and Blue-screen work, it's clearly imaginative and technically good for 1910.