Final Genesis

Then: A bomb explodes, killing the Third Doctor and Brigadier Lethbridge-Stewart. Now: The Doctor's TARDIS arrives with a rising tone, rather than the usual "vworp, vworp", the first sign for Ace and Benny that something is not quite right. Unable to get a straight answer out of the Seventh Doctor, they find the world itself isn't quite right. They see dinosaurs and are arrested by strangely familiar soldiers working for URIC rather than UNIT. The Doctor ingratiates himself into the URIC command; the late Doctor's assistant is familiar enough with regeneration and time travel to work out that although this is a Doctor, he is not their Doctor.