Enterprise reverses course to investigate a geological phenomenon — a planet that's about to be covered with erupting volcanoes. The course change will take the ship near the E.C.S. Horizon, the cargo ship where Mayweather grew up. Mayweather requests a few days off to make a family visit — he hasn't been home in several years, and his father is ill. The ensign is somewhat nervous about seeing his family — his father didn't seem to approve of his decision to leave Horizon to join Starfleet. Ultimately, Mayweather's homecoming proves to be bittersweet — when he contacts his mother, Rianna, she reveals that his father died a few weeks ago.

Once aboard Horizon, Mayweather finds himself adapting to the rough-around-the-edges cargo ship. It's sturdy, but doesn't boast the high-tech perks of the warp-5 Enterprise. Most of the ship's crew welcomes Mayweather warmly, expressing pride at the young ensign's accomplishments. Still, Mayweather can't help but feel a bit out of place. His brother, Paul, was promoted to captain upon their father's death. While Paul acts like he knows what he's doing, Horizon isn't exactly running like clockwork — the new captain doesn't seem to be quite ready for the job. He also seems resentful of Mayweather — when Mayweather attempts to make a few upgrades to Horizon, Paul curtly asks him to stop.

Soon, the Horizon faces a new, potentially disastrous challenge. An alien ship attacks the cargo vessel, attaching a dangerous homing device to its hull. Mayweather surmises that the device could explode if they try to detach it. Also, the crew notes that another ship was attacked several weeks earlier after a similar device was planted on their hull. An alien cruiser stole their shipment and killed several crewmembers. While Mayweather wants to adapt some of Horizon's systems in order to better prepare it for a possible fight, Paul insists that they jettison some of the cargo and attempt to make it to their next stop as soon as possible. If they can't make it, they will simply turn the cargo over to the aliens. Mayweather isn't too fond of this plan, but Paul is firm — this is his decision.

Meanwhile, on Enterprise, Trip is trying to convince T'Pol to come to a screening of "Frankenstein." T'Pol has no interest in human horror movies, but Archer eventually convinces her to go — fraternizing with the rest of the crew will be good for her. To her surprise, T'Pol finds herself engrossed in the movie. In fact, she later tells Archer and Trip that she sees it as parallel to the experience the Vulcans had when they first landed on Earth. She notes that she is looking forward to watching "Bride of Frankenstein."

On Horizon, Mayweather is making a few upgrades without Paul's knowledge. When Paul finds out, he angrily confronts his brother. Mayweather, however, is firm — he's just trying to help his family and the crew. Later on, the ship is approached by an alien cruiser. Paul prepares to surrender their cargo, but then the aliens make another request — surrender the Horizon. Paul enlists Mayweather's experience, ordering his brother to bring the makeshift weapons upgrades online and take the helm. Mayweather suggests cutting the cargo loose and battling the alien ship with Horizon's command module. Paul agrees, and the brothers are able to successfully fend off the alien attack.

In the end, Paul reconciles with his brother, and sends Mayweather back to Enterprise. Pleased that he and Paul have come to an understanding, Mayweather promises to visit soon.