Previously, the Galactica found a planet that had yummy algae, but also the mystical "Temple of Five." Before anyone could blow it up, the planet's sun supernova'd. But the supernova, and various symbols inside the temple, looked a lot like a painting that Starbuck drew on her apartment back home on Nuked Caprica. Which prompted her to dive her Viper into a pretty, swirly cloud mass resembling said "mandala," getting herself blowds up in the process. Or, not, because she came back in the last five minutes of the Season Three finale.
Also during the last few minutes of the finale: The fleet jumps to the Ionian Nebula, and gets attacked by a bunch of Cylon Basestars.
Oh, and, Tyrol, Tigh, Sam Anders, and Tory the Aide are Cylons.
Plus, Gaius Balter is not guilty of treason. Wheee!