The Importance of Being Earnest

 was a show. When Kája saw the Fifth Doctor in his Edwardian clothing, she asked him if he was "in earnest". When he replied that he was, she asked him when would the show open. (PROSE: Suspension and Disbelief)

Behind the scenes
The Oscar Wilde play is spelt "Earnest", but Suspension and Disbelief spells it "Ernest".