The title of the episode is "Not in Portland" and it contains flashbacks for Juliet. You might have heard all that already. It explains how she got on the island.
DO NOT READ FURTHER IF YOU DO NOT WANT DETAILS
A plot summary:
Kate and Sawyer escape and are chased through the jungle. They are saved by Alex, who enlists their help freeing her boyfriend Carl. They make their way to the beach where they can see their original island, and are about to leave in a small boat when they are cornered by Danny.
Before Danny can shoot Sawyer, Juliet steps out of the jungle and kills Danny. She tells Kate and Sawyer to take Carl and leave, but insists Alex stay behind because that is the only way Ben will allow Carl to live. We also learn that Ben is Alex's father (or at least that is what Alex has been led to believe.)
Jack completes the surgery with the help of a queasy Tom (Zeke) and saves Ben's life. Part of the deal was that he needed to hear from Kate that they were safely away before he would continue. When Kate radios from the beach, he insists that she (they) never return for him.
As part of Juliet's story we learn that she was a doctor specializing in fertility and made a breakthrough that is not fully explained other than apparently inducing pregnancy without the need for male and female... contributions. Her work is under the control of her jerk ex-husband, a new character. Her experimental work is being done primarily on her sister, who also appears to be undergoing chemotherapy as she is hairless.
Doctor Julet is recruited by a company supposedly based in Portland, but tells them she can't take the job because her ex-husband/boss won't let her go. She get emotional and tell them she can only take the job if he gets hit by a bus.
A short time later, he gets hit by a bus. The original recruiter arrives, accompanied this time by Ethan Rom, and they entice her to join them... but they are Not In Portland. We later learn that she has since been on the island for three years and a few months. She's eager to leave, which likely explains her depression and tears when we first met her.
During the time while the escape was on, Ben awoke during surgery, heard the escape-surgery situation, and asked for a few private minutes with Juliet. After that conversation, she pleaded with Jack to save Ben, and then went and aided the escape. At the end of the episode Jack asks Juliet what Ben had said that changed her mind. She tells Jack that if Kate and Sawyer escaped, and Jack saved Ben, that Ben would allow her to finally leave the island and go back home.
The episode does push the plot along, fills in some answers, doesn't raise too many other mysteries. The question of Alex's parentage is sure to be an issue down the road, and who knows what becomes of Carl when Kate and Sawyer get back. Maybe he can bunk with Nikki and Paolo. 
