By the power of truth, I, while living, have conquered the universe.
Wow, the majority of people here know nothing about the story. I'd like to point out that ODST is set during the events of Halo 2, not Halo 3. It picks up from the ending of the mission 'Metropolis'.
Buck was under the impression that they'd be infiltrating Regret's Carrier as it hung over New Mombasa, but Dare was actually on a classified ONI mission to divert their pods away from the Carrier and retrieve Doctor Endesha's (from Sadie's Story) data on the seismic activity under the Earth from the Superintendent's core.
Regret jumped into slipspace to go to Installation 05, the pods were thrown off-course and a portion of Truth's fleet arrives to begin the Great Schism against survivng Elites and attack the humans.
The seismic activity was actually the Portal Artefact leading to the Ark, the Covenant Loyalists come along at the end of the game and begin glassing that area to uncover it.