In editing, it was discovered that although parts one and two overran significantly (resulting in a number of minor edits), episode four could not be made to stretch to its requisite duration. Because of the structure of the third installment, it would be difficult to move up material to help fill the gap, as was the normal practise. Consequently, Eric Saward was forced to ask Christopher Bailey to provide four minutes' worth of extra material featuring only the regular cast to pad out the episode. Bailey developed two scenes set in the expedition dome's airlock, one involving Tegan and Adric discussing Hindle's bomb, and another in which the Doctor assures them that Hindle is no longer a threat. These two scenes were recorded during the making of Earthshock, Peter Grimwade's next Doctor Who (1963) serial.
This episode was watched by 8.9 million viewers on its original transmission.