Troilus and Cressida

 was a play by William Shakespeare. The Eighth Doctor once feared that he would create a temporal paradox by accidentally taking Shakespeare to visit the real Troilus and Cressida (his former companion, Vicki). Thus, he asked Charley to keep Will totally drunk during their visit to Asia Minor, so that Will could not remember the events, and thus integrate them into his later play. However, as the drink wore off, Shakespeare revealed that he already knew of the tale of Troilus and Cressida from the works of Geoffrey Chaucer. Meanwhile Charley reminded the Doctor that Shakespeare's play ended in a tragedy for the lovers — something which wasn't true of the very happily married Troilus and Cressida. The Doctor was then forced to admit that he had never read Shakespeare's play, and so was being over-cautious without reasonable cause. (ST: Apocrypha Bipedium)