The Doctor starts to leave with the TARDIS, all alone, because Dodo sent a message that she wished to stay in London. Except, Ben and Polly have accidentally boarded the TARDIS just before dematerialization. Once he sees them he's knowing that they're stuck with him for awhile, but of course they're obstinate that they're in 1966 London and that Ben has to get to his ship, etc. Then the ship lands somewhere and, despite now being on a beach, still thinking they're in 1966 they start heading towards the church they see in the distance. And thus ensues an Adventure in the past.