Summary of what's happened so far:
First off, this takes place seven years after the series.
The story opens with Darkwing going to fight Negaduck. During his encounter with his evil double, Darkwing becomes extremely injured; so injured that he cannot fight crime for several months until he's completely healed.
Her father now down and out, Gosalyn takes over as St. Canard's crime fighter, donning her father's mask, cape, name, and persona every night. She works at this for a week or two and finds that she has a gift for beating bad guys. Negaduck hears of her success, and decides to meet her himself.
The meeting doesn't go as Gosalyn (now Darkwing Duck) planned; she gets knocked unconscious and Negaduck takes her back to the Negaverse. Negaduck's plan is to hold Gosalyn hostage so Darkwing will come to save her and, once both Darkwings are in the Negaverse, he will kill them both and have St. Canard for himself.
Just one problem: Darkwing can't fight crime let alone go and save his daughter. So, Gosalyn has been stuck in the Negaverse for about a month and a half at this point, and Negaduck is finding, with increased loathing, that he likes the teenage girl.
Then, one day, Gosalyn admits to feeling lost and alone. That is too much for Negaduck; upon finding out that she is going through the same emotional stages he went through as a teenager, he lashes out and leaves to try and conquer his past.