The prequel follows the outbreak in Seoul Station, or really the city itself where we follow Hae-Sun, a former homeless girl who, after a row with her sleazy boyfriend, ends up in the middle of a zombie horde attack. While Hae-Sun is running for her life her father and her boyfriend race against the clock the find her.
While not great as an animated zombie film Seoul Station isn't in all a bad film. The zombie action is well put together and the animation is fine, but as a prequel I felt a little detached. The feel of Train to Busan is totally lost in this film, the voice acting is tedious and I wasn't feeling the story at all. The first part of the film between the old man homeless guy and his odd little friend running around the city was drawn out a little for me and I didn't feel any connection with the characters in this, unlike the connection you feel in the main film.
Seoul Station is one of those animated features that would fit better in the special features section of the Blu-Ray of Train to Busan, but it is very much watchable I just would wait for it to go down in price a little first.