In Star Trek: Horizon, the narrative unfolds in a time before the establishment of the United Federation of Planets, focusing on a coalition of planets led by Earth as they confront the Romulan Star Empire. The story is set during the era of Star Trek: Enterprise, primarily following the crew of the USS Discovery (NX-04) under Captain Hawke.

The film begins with a prologue set 250,000 years in the past, depicting a war between two ancient races, the Arioui and the Iconians. As the war reaches its climax, some Iconians transport a planet harboring a powerful weapon to safety to prevent it from falling into the wrong hands. This planet is named Horizon.

In the present timeline, the Discovery is on a critical mission to return T’mar, a Romulan defector with vital information, to Starfleet Command while being pursued by Romulan ships led by Admiral Verak. After a fierce battle, they narrowly escape with assistance from the USS Enterprise. Upon their return, T’mar reveals that the Romulans are constructing a devastating super weapon near Earth.

To thwart this threat, a fleet consisting of Vulcan and Earth ships embarks on a mission to locate and destroy this weapon. During their attack, the Discovery inadvertently creates a spatial rift that transports them to the hidden planet where Horizon was taken. Here, they encounter Daekon, a Romulan defector from the future involved in the Temporal Cold War.

The crew must confront Daekon and ultimately destroy the weapon, which concludes their conflict with the Romulans and resolves lingering tensions from the Temporal Cold War. This film serves as a bridge between Star Trek: First Contact and Star Trek: The Original Series, enriching the lore of the franchise while delivering action and drama typical of Star Trek storytelling