Ravensbeck River Police operate in a quiet backwater on the Thames. Their nominal chief, lazy Sarge, spends all his time fishing and the real boss is the Prof, who, with young assistant Darwin, is amazed to find a number of ducks and geese dyed a variety of colours. Pollution is suspected but Prof discovers that a baker's little girl has been emptying his colourings into the river. Temporary policewoman Marilyn, the object of Darwin's affection, is annoyed when hunky Hero is given the permanent posting she wanted but Hero does save Sarge when he falls into he river.