Forced out of their rented rooms in Hammersmith, loving couple Jack and Peggy find larger accommodation - and their first shared home together - in the form of a houseboat, moored a little further upstream on the Thames. The pair are quick to snap up the property from its present tenants but fail to enquire after the final details, and soon discover that the owner of their mooring is no jovial landlord but in fact quite the cad, proposing to double their fee, and add on extra charges for good measure.

Freshly married ahead of moving into their new accommodation, mechanic Jack is keen to give his young bride a happier honeymoon and a conversation with regular customer and friend Sidney Randall leads the duo to plot a trip down the river aboard the new 'pleasure yacht'; a surprise for Peggy and a chance for Sid to spend some leisure time with his stripper girlfriend, Sandra.

After a few repairs - not least to the engine, and starting it up for the first time in at least three years - Sid and Jack get the boat on the move, and the quartet are soon heading down the river at a rate of knots, much to the annoyance of other users of London's central waterway! Heading for Kent, the trip goes awry when their intended destination turns out to be little more than a grim industrial wasteland, and as they begin plotting a bearing for Ramsgate, thick fog quickly sets in...