The title depicts a viral outbreak on a luxurious passenger ship and is set in December 1998, during the events of Resident Evil Code: Veronica. It introduces an underground organization established to put an end to the global operations of Umbrella, the company responsible for the Raccoon City disaster. Leon S. Kennedy, one of the protagonists of Resident Evil 2, joined the initiative and received orders to investigate the ocean liner Starlight which is rumored to be carrying a new type of bio-organic weapon (BOW) developed by Umbrella. Eventually, the headquarters lose contact with him and Barry Burton, a support character from the original Resident Evil, is sent in to find his whereabouts.
Source: Wikipedia, Resident Evil Gaiden , available under the CC-BY-SA License.