When a bank client's debt means his rowing boat must be auctioned, Mainwaring can't resist acquiring it for river patrols, however overstretched his men already are. Frazer supplements the nautical novice's Seascouts manual 'knowledge', and becomes the coxswain. But Mainwaring disregards all warnings, so they row through mist straight into sea. Hearing strange voices, they assume they are in Nazi-occupied France and hide, until they wake in a moving train.
