On second thought:
Do a retread of "Yesterday's Enterprise."
24th century: USS Enterprise-B, commanded by Jean Luc Picard, is going on its maiden voyage and seen off by Admiral Kirk (played by William Shatner, of course). Old Kirk gives Picard a very cryptic order: go to the Khitomer system and scan for temporal anomalies. Khitomer is the capitol world of the Klingon Confederacy, so he's confused about these orders.
Flashback 40 years:
Romulus invades the Qonos and curb stomps the entire empire, devastates their fleet, proceeds to glass their planet and then the outlying colonies one after another.
What's left of the Klingon fleet gathers at Khitomer, which becomes the last safe haven for their refugees. Starfleet sends the Enterprise to "observe" the situation, partially to give Starfleet eyes on the conflict but mostly to make sure the Klingon refugees don't go fleeing into Federation space. A small Romulan fleet gathers to attack the colony, and the klingon defenders are loosing... so Kirk decides, on his own initiative, to fly into the battle and help the Klingons.
Plot twist: the battle with the Romulans opens a temporal anomaly as predicted, and Kirk orders Picard to go through it. Enterprise-B arrives in time to save Enterprise-A and the two starships save the entire colony plus twenty million refugees. Picard tells Kirk that he's here because his older self ordered him to come here, and now Kirk knows that 40 years from now he's going to have to send his successor to reinforce himself on his maiden voyage. And the Klingons, touched and impressed by Starfleet's cleverness and compassion for a people who don't even like them, warm to the idea of joining the Federation and become full members a decade later.
And then the canonistas' heads explode.