Johns stated during the panel that has no interest in a story where Superman and Dr. Manhattan just fight each other, that most of the questions people have regarding Rebirth and Watchmen's connection to it will be answered and says you only need to read Watchmen and have a general understanding of Superman and co to follow the story. There will also be no tie-ins or crossovers with other books.
Summary of the first 6 pages:
-- The story begins in the Watchmen universe in November 1992, 8 years after the events of the book.
-- Rorschach's journal was read, verified, and published. Ozymandias is now the most wanted man on Earth, charged with murdering 3 million people. No one can find him, Nite Owl, or Silk Spectre.
-- The European Union has collapsed. The Vice President is holding hostages in the White House. Russia believes the US government was a willing participant in Ozy's plan. North Korea has nukes. Americans are fleeing to Mexico.
-- News stations are shut down and are replaced with a state-run broadcast with William Buckley Jr. Jr explains Russia has invaded Poland and they have four hours to withdraw from the country before the bombs drop. A mandatory evacuation of the US has been issued.
-- The pages end at a prison, where the inmates are attacking a guard for his keys. Rorschach appears and takes the keys for himself.
The pages can be seen in full at the source. The first issue of Doomday Clock will be released November 22.