 The effect was introduced in "Day of the Daleks" (1972), when Jo Grant asks the Third Doctor why a group of time-travelling guerrillas on a mission to assassinate a diplomat cannot simply go back into the past and try again if they fail.In reply, the Doctor cites the principle and begins to explain, but is interrupted before he can go any further.The "Effect" was invented by script editor Terrance Dicks and producer Barry Letts to gloss over the plot problems inherent in the time travel premise of the serial.