Contrary to popular opinion, the end of the series did not arrive unexpectedly. The first episode of the fifth season was the last episode of the season, and was plotted to provide an opportunity to develop either a Farscape movie, a return of the series, or - as it turned out - a scifi mini-series.
The end of the fourth season left Moya's crew in a convoluted cliff-hanger and about 1/2 way through what was probably intended to be a lengthy, complex story arc. The show had gotten back onto its feet after a weak fourth-season beginning. Some of the characters which had been dispensed with by Scifi's chronic mismanagement of their creative teams (note also what they did to MST3k and Sliders, and what they will probably do to Battlestar Galactica as it winds down) had been brought back in. From the perspective of quality entertainment, everything was looking good for season 5.
Then Scifi withdrew its funding. Popular outcry about the cancellation was pretty immediate. No spoiler here, but I need to give some indication of where the episode leaves you to continue this train of thought. I don't recommend reading the following unless you have already seen at least some of the rest of the series.
The episode features John and Aeryn working and loving together like they never have before - on a mad scramble to reach earth before the Scarrans. Can they trust Scorpius, Sikozu and Bracca's offer to help? Will they be able to go it alone? Will the effort kill them? Will earth survive a possible Scarran invasion? Crichton, who has just killed thousands of people with a nuclear blast at Scarran military headquarters (in a previous episode), is darker, stronger, and more serious than perhaps we have ever seen him. And Aeryn has changed dramatically as she falls even more deeply in love with him.
Wisely... the last episode leaves the door open for either an end or a continuation - but also holds out the promise of continuation very powerfully. It appears to me that the cancellation of the show's fifth season required the creative team for the show to accelerate the story arc they were working on and to resolve much while leaving their options wide open. As always, Farscape did what it intended with class.
To get this wild, light-speed, episode, you don't just need to know the series and its characters, you need to be wide awake.
Get the DVD box-sets and dive in. You've got a lot of work to do and the journey is well worth the effort!