Dr When and Dr Where haven't learnt their lessons well 
and are on the verge of failing their exams.
Using an earlier model of Dr Who's Tardis, they set off
on an exploration of the 20th century.

Back they go to the start of the century

They meet the Jarrow Marchers

and are joined on their travels by Jack

 The 1920's = 'Singin' in the Rain' and the Charleston

Then on into the 1930's

 World War II, the blackouts, air raid wardens, evacuation...

 and the GIs