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So basically, Intervention Earth left off on Omega's return, Romana III (Trey), Narvin and Ace stranded in a broken TARDIS, and then Braxiatel appearing and saving Trey. And war is on the horizon - Time War, or war with the temporal power, or Omega - is never clarified - but it's gonna be big.

Narvin and Ace are found by the Timelords, but executed for their (unintentional) part in Omega's return.
Trey tells Brax to travel back to just before her regeneration, save her, and forewarn her past self (Romana II) of Omega's plan, so she can thwart them before they occur, instigating a paradox.

Off-screen, all Omega's followers are arrested, and that's the end of that.

Leela was tricked inside a teleporter by Romana during the events of her regeneration (Romana is counting on the energy released from a regeneration to stop a critical mass exploding and causing a massive spatial-temporal singularity) - which originally caused her to be stranded on a distant planet and never rescued or in contact with the time lords ever again. However, because of the paradox, Brax and Romana also picked up Leela - so now Leela also exists as a paradox.

There's also this Time Lady ghost called The Watchmaker, who basically functions like the bug things from Father's Day and tries to correct paradoxes...by murdering people. Except when she doesn't.

Blah blah blah, story stuff happens and is pretty okay...then Narvin and Ace are murdered by the Watchmaker. Romana ends up meeting paradox Leela (who is now old), then they meet the Watchmaker who tells them they're both paradoxes, and what will happend to Gallifrey if they allow the paradox to continue i.e:

Romana regenerates into Trey, Singularity averted, Leela is lost in space => events of Intervention Earth => Omega => Gallifrey destroyed.
Or
Romana saved by Brax, Singularity occurs, Leela found => events of Enemy Lines (which I've skipped over but involves war with the temporal powers) => Gallifrey destroyed.

So rather than do anything clever, they just create another paradox (which is apparently fine by the Watchmaker!?), and travel back to the moment just after Brax rescued Romana, and avert the singularity by regenerating in her younger-self's place (and dying? It's not clarified), and old paradox Leela dying to correct the paradox of two Leelas existing at once.

This basically creats a timeline in which Romana is saved by Brax and never rengerates into Trey, Omega doesn't return, the singularity is averted, and everything is tickety-fucking-boo.

Then the characters are like "Huh, that was weird. Wonder why that happened? Guess we'll never know".