The secret code

There was a bit where some characters found a secret code from their parents hidden in a letter. If you took every second letter and read it down vertically, it spelled out a message, and continued to every third letter and then every fourth. It was very impressive that it made sense both in the normal horizontal way and the secret vertical way.

There was also a scene from a Doctor Who episode where the Seventh Doctor was stuck in a school filled with Dalek mutants (not in their cases, just giant green blobs. They were surrounding the TARDIS outside and covering the exits from the school. The Doctor managed to get upstairs and out onto the rooftop, luring them out to follow him. He then got down to the ground and ran to the TARDIS, which was now clear from Dalek mutants. In the dream I knew this was part of a special episode that featured several Doctors.