The United Federation of Planets has been in conflict with the Klingon Empire for decades.
It's about to get real.
On its way back to Earth, the starship Excelsior witnesses a huge explosion on the Klingon moon Praxis—a key energy production facility. Confused, Captain Sulu notifies Starfleet of the strange event.
It turns out that an environmental crisis is unfolding on the Klingon home world and is threatening to topple Klingon society. Spock, the former first mate of Enterprise, has been secretly negotiating with the Klingon Chancellor Gorkon to build peace.
Here's the kicker: Spock has volunteered Captain Kirk to be the Federation's lead envoy.
There are two big problems with this. First, Kirk hates Klingons. Second, the crew of Enterprise is about to be decommissioned after this mission, which makes Kirk even less content with the order.
Regardless, Kirk has no choice but to follow the command. Enterprise meets up with Gorkon and company, and Kirk invites them over for dinner. It's awkward. General Chang, one of the Klingon leaders, is openly hostile to Kirk. On the bright side, Chancellor Gorkon proves to be an empathetic guy who's sincerely devoted to peace.
Then something shocking happens. Out of nowhere, several torpedoes seem to launch from Enterprise, crippling the Klingon ship. In the aftermath, assassins beam aboard and murder Gorkon. Yikes. Although Kirk and McCoy try to save Gorkon's life, Chang blames them for his death and takes them into custody.
After a sham trial, Kirk and McCoy are sentenced to life in a labor camp. Meanwhile, Spock takes control of Enterprise and begins an investigation into the assassination, believing the killer to be aboard the ship.
The labor camp is as bad as you'd expect: our heroes must both perform backbreaking labor and rumble with nasty alien criminals. In what seems like a godsend, Kirk and McCoy meet a fellow prisoner who helps them escape the prison. This prisoner is revealed to be a Klingon plant, however, hired to put Kirk and McCoy in a compromising situation in which they can be murdered. Gasp.
Spock beams Kirk and McCoy aboard Enterprise at the last second, having tracked them ever since they left the ship. He reveals that he's come closer to finding the assassin, and he says that he's learned that the torpedoes were not fired by Enterprise—they were fired by a cloaked Klingon warship.
Finally, the assassin is revealed to be none other than Lieutenant Valeris, Spock's mentee and his choice as his successor on Enterprise. Valeris is part of a conspiracy along with General Chang and several members of Starfleet, and they're planning to assassinate the Federation President at a peace meeting later that day.
Enterprise rushes over to the peace meeting and battles General Chang. At the last second, Excelsior arrives, giving Enterprise the space it needs to take Chang down.
The battle won, the crew rushes down to the surface and stops the assassination. Kirk gives a big speech in favor of the peace talks and shares a bonding moment with Chancellor Azetbur, daughter of Gorkon and new leader of the Klingons.
Back aboard Enterprise, the crew is ordered to return to Starfleet for the last time. That's the biggest bummer ever. Ever defiant, however, Kirk decides to take Enterprise for one last joyride before turning her in.
Atta boy, cap'n.