Earth finds itself threatened when SG Command learns that an asteroid is on a collision course with the planet. The Asgard can't interfere because it's a natural disaster, and the Tok'ra are on the run. SG-1 fixes a crashed cargo ship and intercept the asteroid with a bomb. Things go from bad to worse as their engines cut out, they all but crash, and the air leaks out.
Making things even worse, they discover a store of Naquadah (sic) in the asteroid's core. The Gou'ald towed the asteroid into position and shoved it toward Earth. If the bomb detonates it'll ignite the Naquadah, causing a nova-like explosion. Meanwhile the bomb has been hit by a rock, and Jack must make a quick disarm attempt. Well past the fail safe point, they have only one option: use the wrecked cargo ship's hyperdrive to blast the entire asteroid thru Earth just before it impacts Earth. They pull it off with just seconds to spare.