Well, movies are still "episodes," right?
Star Trek IV.V: The Great Experiment
Less than an hour after the end of The Voyage Home, the USS Excelsior--which has been undergoing simulation testing ever since the events of The Search for Spock--finally heads out on a mission to properly stretch its new transwarp engines: to return to the center of the galaxy, which the Enterprise visited in "The Magicks of Megas-Tu." But while the Enterprise only got there via careful navigation of a temporary natural space-warp phenomenon (long since dissolved), Excelsior will get there under its own power. Except: when Excelsior finally goes full transwarp and hits Warp 10, telemetry is lost.
When Kirk and company hear about this, they immediately attempt to follow Excelsior's route on a rescue mission--but the engines mysteriously break down before they can engage full transwarp, with which the Enterprise-A is also equipped.
So Starfleet sends the next best thing: a fast rescue/tender ship built for just such an occasion, the USS Nuff Said. (Named after the famed half-human ambassador who resolved the Umblenkchoo Crisis, of course.) At high warp, it picks up the Enterprise and drags it along in a tractor beam--and somewhat further along, they find the Excelsior, drifting without power. Its engines are burnt out, and Captain Styles and his crew have inexplicably turned into lizards.
Excelsior's computer logs indicate that the ship reached the core of the galaxy--and bounced off a gigantic energy barrier that wasn't there twenty years ago. After further investigation, Spock realizes that the barrier was created by the interaction of the Excelsior's transwarp field with the technobabble fields of the galactic core. Because of our own technology, the center of the galaxy can no longer be reached.
The Nuff Said grabs the hulk of the Excelsior, and everybody heads back to Federation space--only to be attacked by a decloaking squadron of Klingon ships! For there will be no peace as long as Kirk lives--and furthermore, Klingon spies have reported that another Genesis torpedo is aboard the Excelsior! Kirk sputters that that's impossible, but Spock quietly tells him that it's true. Sealed records in the Excelsior's logs indicate that once they'd reached the galactic core, the mission plan was to use Genesis on an existing planet there, and set up the result as a Federation beachhead.
The Klingons attack, and there's no way for a tender and two broken ships to hold them off--except by using Genesis as a weapon. Which they do. The Klingons are destroyed, but all three Federation ships are badly damaged. Genesis is clearly too unstable to use even as a weapon.
We go home. The transwarp engines are to be stripped from both Excelsior and Enterprise, which will go through extensive refitting. The end.