Seska was one of the crew of Chakotay's Maquis ship when it was pulled into the Delta Quadrant by the Caretaker's array. During the course of the show she appeared as only a minor member of the crew but there was a hinting that there might have been some closer relations between her and Chakotay.
After some Federation technology is found to be on board a Kazon ship, an investigation leads to her being discovered to actually a Cardassian operative. She quickly arranges for herself to be beamed to a Kazon ship and joins them. She takes part in capturing Chakotay and while she has him in her custody she takes a sample of his DNA and uses it to have a baby. This was a set up to draw Voyager into a trap.
The opening move of the plot to capture Voyager included infiltrating a Kazon onto the ship with a explosive compound hidden in his blood. Using the hatred of Voyager that was created by a botched attempt at a peace conference brokered by Janeway between the Kazon and the Trabe she was able to use the various Kazon Clans to cause specific damage to Voyager's Auxiliary Command Processor in order to make the use of Self Destruct impossible. Their operative then exploded himself to help disable the ship. This made it possible to board and capture the ship. They then marooned the crew on a desolate planet and left them to fend for themselves. This led to the death of at least 2 crewmen because of Neelix's stupidity. After taking over the ship, The Doctor examined the child that was suppose to Chakotay's and found it to actually be Cullen's.