The Five Doctors

From: http://www.bbc.co.uk/doctorwho/classic/episodeguide

The Doctor's past incarnations are taken out of time by a forbidden time scoop device. The fourth Doctor becomes trapped in the vortex but the others find themselves, together with a number of their old companions, with the fifth Doctor and his companions in the Death Zone on their home planet Gallifrey.

Here they face a Dalek, a Yeti, a quicksilver Raston Warrior Robot and numerous Cybermen. Also present is the Master, who has been summoned by the High Council of Time Lords to help the Doctor. It turns out that President Borusa is the mysterious operator of the time scoop. He aims to use the Doctors to breach the defences of the Dark Tower - Rassilon's tomb - so that he can enter there and claim immortality.

When he does so, however, he is condemned by Rassilon to eternal existence in the form of a living statue.

9 out of 38 (23.7%) raters say this story requires a previous story.