When newlyweds Jack (Ian Carmichael) and Peggy (Janette Scott) face eviction, they are tricked into buying a rundown houseboat. After rebuilding the engine they take their old friends Sid (Sid James) and Sandra (Liz Fraser) on a local trip down the river to Folkestone. But as the fog gets thicker and mishaps become funnier, they find themselves chugging into a harbor in France. With little fuel or supplies left they resort to desperate measures to sail their way back home.