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.

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?