This sequence is mostly exposition. Pongo and Perdita learn that there's news of their puppies. They meet the Great Dane and learn that Cruella was responsible for kidnapping the pups, then they travel to rescue their children.
The dogs are faced with various hazards on the journey, several of which will play a part on the return trip. They have to dodge a truck (and a truck will later take them back to London) and there is falling snow (which will end up revealing them to Cruella near the film's climax). They also have to swim through icy water.
It's interesting that writer Bill Peet decided to establish the length and dangers of the journey only briefly. Rather than dwell on the details, the sequence is short so that the story can get back to the happenings at Hell Hall.
Once again, sequence director Woolie Reitherman relies heavily on re-use, though it's not very obvious. Shots 3.1, 3.3, 12, 12.1, 12.2, 14 and 14.2 all use animation from earlier in the film.
The best parts of this sequence are the layouts, supervised by Basil Davidovich, and the backgrounds. A great deal of this sequence is done in long shot, marking the shifting environment and the distance between London and the countryside as the dogs travel to their puppies.