Nothing much happens. They blow stuff up in a CGI spacebattle that doesn't impress any viewer. Long speeches about the horror of war are made. Nothing much else happens. The Chig's attack Earth but like all bad guys they miss by a mile and end up blowing up Pluto.
Nobody cares as it was a boring planet.
Meanwhile, West and Cooper and bunch of dialogue-free cannon fodder characters attack the Chigs while bad rock music plays. They save the day and get forgiven for Roundhammer. However Commodore Ross the Commander of the Saratoga has mysteriously vanished since the end of season 1 and hasn't been seen or mentioned since, only nobody seems to have noticed. The Silicates show up again and act tough. Everybody laughs, the Silicates go away and vow to get real jobs.