Buffy and her sis walk through the park when Impmon 'attacks' them. Buffy then proceeded to whup Impy's ass in the background while dawn sat on a bench and mopes until Calumon shows up.

The Dark Whirlpool, coupled with a spell gone awry, lands Buffy and friends in the Kaiser's base during episode 19 of 02. Things go downhill from there. BtVS/Digimon 02.