Though replete with additional nuance, the nub of the narrative problem is easy to grasp. Mawdryn Undead tells viewers that the Brigadier retired from UNIT in 1976. However, the events of The Invasion, the first story in which UNIT properly appears, approximately occurred in 1979. Thus, the UNIT dating controversy is, broadly speaking, an attempt to understand how the Brigadier could have retired from UNIT before UNIT even existed. Writer Ben Aaronovitch has notably opined that there is simply no way to retcon the problem.