So, in #12, they decided to save us the trouble of having #13 do "The Search for Kirk". So that puts #13 as...
Kirk violates the prime directive again, resulting in yet another five-minute demotion and a random crewmember reassigned to a garbage scow for the Ganymede outpost.
Suddenly an alien probe starts microwaving Earth's oceans. To save Earth, Starfleet instantly promotes Kirk to double-plus-admiral and gives him an experimental portable time travel module, which he uses to take the enterprise back to 1980s Earth.
Once there, he must find and kill a 10 year old Benedict Cumberbatch before he invents the plague that wiped out the whales.
In the second to last scene, wacky hijinks ensue as we learn that Uhura secretly hid a chihuahua in her purse before returning to the future, which due to tachyon flux has evolved into a catch-phrase spewing mascot with the power to float just out of reach.
Finally, Kirk makes a speech (possibly as a voiceover) intended to beat some cheesy moral principle about the benefits of communism into the audience.