It is nearly 45 years since Maureen O'Brien and Peter Purves first played these roles, yet they sound no older than when they appeared in black & white! That is the main delight of this story, and they fitted together as though they had never left the show. Having two narrators necessitates splitting the story into two - Vicki's & Steven's - with them being rendered asunder early on and only reuniting towards the end.
Unfortunately the plot, starting very much like an old Quatermass story, does not hold together as stongly as the cast, with the idea of joining an alien entity with the suffragete movement feeling very forced. There are some lovely points, such as Steven and a skeleton on a London omnibus, and Vicki becoming involved in a suffragete riot in London. And both manage to convey the First Doctor's endearing mixture of gruffness and curiousity.
If you enjoy William Hartnell's version of the Doctor, then this is an enjoyable 90-odd minutes, which nicely filled a midweek drive back down the M1. Otherwise, a decent idea that doesn't quite fulfill its potential.