Voyager encounters a damaged and adrift Malon freighter in danger of releasing deadly theta radiation into that sector of space and rescues two crew members. Captain Janeway sends a repair crew to try and contain the radiation spread. Before they embark on the mission, the two crewmen warn them about the "Angel of Decay" that wreaks havoc aboard the ship.

Captain Janeway recalls a 21st-century ancestor Shannon O'Donnel, involved in creating a seminal self-contained biosphere called the Millennium Gate on Earth on New Year's Eve 2000. O'Donnel must convince the last holdout, local bookstore owner Henry Janeway to approve the plan.