In the 24th century, the starship USS Enterprise and its crew are dispatched to intercept an extraterrestrial threat: the Borg, a collective of cybernetically enhanced beings intent on assimilating Earth into their hive-like consciousness. The Borg has created a time vortex and traveled back to the mid-21st century to prevent a historic event — the first successful test of faster-than-light travel by humanity — which would initiate Earth's first contact with an alien species and lead to the future in which the United Federation of Planets flourish. The Enterprise pursues the Borg through the temporal vortex, emerging in Earth's orbit just days before the critical test flight.

On the ground, the Enterprise crew finds a world still reeling from the effects of World War III, and they must locate and assist the flight's brilliant yet unconventional inventor to ensure the test goes ahead. Amidst this, the crew faces the challenge of the Borg's presence on Earth, who are in the process of altering the timeline from the planet's surface. The crew splits its focus; some engage in a desperate battle against the Borg onboard the damaged Enterprise, while the rest unite to protect humanity's pivotal moment on the ground, understanding that the very existence of their peaceful future is at stake.