This show is impeccably well-researched, and Ms Hartshorne's impressions are eerily, uncannily accurate.
Dietrich's voice is easy to parody, less easy to tribute, but Ms Hartshorne treads this gravelly path with panache.
The impression, she says, was honed while teaching students with behavioural problems. "With those pupils, a lot of the time it's a softly, softly approach," she said.
"But sometimes I start singing 'Falling In Love Again' to them, in a very low register, dropping my eyelids and becoming a different person.
"When I suddenly go into a Marlene, they think, 'Oh, she could be frightening.'
"It shuts them up."