Enredo

In a time prior to the United Federation of Planets, a young coalition of worlds led by Earth battle the Romulan Star Empire for their very survival.

The film is set in the era of Star Trek: Enterprise, with the NX-01 even putting in a brief cameo, but it's the NX-04, the Discovery, Commanded by Captain Hawke (Paul Lang), and her crew is the primary focus of the story.

There are numerous tie in's to the different Star Trek series' which bring Horizon into canon. Horizon fills the void between First Contact and TOS regarding the "Federation's" conflict with the Romulans.

The film opens with a prologue 250,000 years earlier, during the last moments of a war between the Arioui and the Iconians. Some of the last remaining Iconians manage to Stargate away a planet with a massively powerful weapon and hide it from the Arioui, who, it is implied would have used it to rule or destroy the rest of the galaxy. The planet's name is Horizon, and those last Arioui keep it hidden, lest it fall into the wrong hands.

The first scene in the present time begins with Discovery racing back to earth to get T'mar and her secrets to Starfleet Command with several Romulan ships Commanded by Admiral Verak (Ryan Husk) in pursuit. In a running battle with a trio of Romulan Warbirds. The Discovery has a newly discovered Romulan defector, T'mar (Callie Bussell) on board and the chasing Warbirds want her back - dead or alive.

They take heavy damage and suffer casualties. The Enterprise makes a cameo here, swooping in to save them at the last moment. Captain Archer's ship makes several appearances throughout the movie, which Enterprise fans will love. Discovery limps back to space dock to undergo repairs and debrief.

After an all-too narrow escape it is found that T'mar is a wealth of tactical knowledge and revels that the Romulans are building a weapon of mass destruction. Thus the stage is set, and the Discovery is set out to find out the truth of what the Romulans are up to and to prevent them from gaining an upper hand in the war.

They learn from T'mar that the Romulans are building a super weapon just two light-years from earth. A fleet of Vulcan and earth ships, including the newly repaired Discovery, head out to find and destroy the weapon, called Horizon. The fleet comes under attack by Romulans, and it's left to the Discovery to destroy the weapon with their new prototype torpedoes. The torpedoes cause an unexpected spacial rift to form, and Discovery is pulled through.

The crew find themselves at the planet Horizon was brought to by the Iconians. While investigating a structure on the surface they are found by Daekon (Rocco Guirlanda), a Romulan defector from the future and a combatant in the Temporal Cold War. We learn that he is the mysterious man from the future in the Enterprise series. The crew of Discovery works to defeat him and finally completely destroy the weapon, bringing the Temporal Cold War to a close at long last.