Jazz-loving, woodship teacher Trevor Chaplin and his girlfriend Jill are on the trail of a saleswoman who mistakenly delivered a set of records that were supposed to be by Bix Beiderbecke.

Jill is detained by the police, Trevor goes in pursuit of a platinum blonde, Big Al reveals some of his secrets, and Sergeant Hobson tries to make sense of it all.

Trevor has succeeded in finding the music he's been looking for and Jill's campaign is reaching fever pitch. But who is the mysterious supergrass and the woman 'with a past'? Will the truth ever be revealed to Det Sgt Hobson?

Trevor is at the apex of an emotional triangle, Sergeant Hobson is in trouble, and Jill is taken out to dinner.

Jill and Trevor have an emotional crisis, Sergeant Hobson has a policing crisis, and Big Al has a storage crisis. But the question on everybody's mind is: 'Who is McAllister?'.

Somehow, a bizarre twist of fate plunges the couple into a mystery involving bureaucracy and death threats. All is revealed in this final episode: will Hobson find fame and glory, will Trevor and Jill find true happiness, and will Mr Carter ever really understand his two teaching colleagues?