Basically Katniss accidentally explodes the arena because of how the other guys rigs his electrocution machine. Then the rebel planes come in and rescue Katniss and Finnick, but the Capitol planes take Peeta and the rest of the tributes.

Then, the Capitol, trying to show that they can easily crush Katniss and the rebels, flies to District 12 and destroys it using fire bombs like they did District 13. (Hence the ending quote, "There is no District 12.")