I've never seen the Batman Beyond series, but that's not really required to watch this film. You'll catch on quickly enough and when all is said and done, this is just another face-off between Batman and the Joker. It feels like the hundredth iteration of the same thing, then again I'm not a Batman fan to begin with.
The film's TV roots are painfully obvious. Character designs are very plain, the animation looks cheap and unappealing. It feels like two episodes stuck together, with little extra budget to make it into something more. All that's left then is the plot, but that's just the usual mishmash of cartoonish nonsense and dim drama.
You really have to be a big Batman fan to get anything out of this film. It's TV-level cheap, with atrocious dubbing, lame animation and uninspired direction. That seems to be a returning issue with many of the animated Batman films, then again they get pretty decent ratings so someone must like them.