Betsumon are eccentric, lanky Digimon in Gatomon bodysuits who move around in large packs. Despite their silly appearances, they are dangerous Digimon who impersonate other Digimon or humans and replace them in their lives, rendering their victims completely unrecognizable over time. When disguised, they neither look or act like their victims, but people around them will recognize the Betsumon as the original, even if they haven't met the Betsumon or made eye contact with them. Eventually, the victim will vanish from existence. Digimon who are removed from existence by the Betsumon do not leave a Digi-Egg upon "death". They tend to gloat on their victims once they successfully steal their lives and often chuckle even if they are defeated.
Before the start of the episode, a Betsumon impersonated Jellymon when she was out managing goods for other Digimon. Another one approached Hiro in a Gammamon bodysuit when Gammamon was out to meet his new friend, a Gotsumon, for a play-date. Unfortunately, his new friend was a no-show and the Betsumon infiltrates Hiro's dorm, integrating itself to his daily life. Hiro still recognizes Gammamon until the next day, believing that he was always in his room. However, after a Betsumon implied to be the pack leader impersonates Gotsumon and tricks Gammamon out to wait for him, Gammamon becomes entirely unrecognizable. Clockmon fails to recognize him, Ruli and Angoramon cannot over a webcam, and Kiyoshiro doesn't recognize him even if he never laid his eye on the fake Gammamon. Suffering from an identity crisis, Gammamon meets another one of their victims, a Meicoomon. After stating what happened to him, the feline Digimon vanishes from existence, and Gammamon crawls back to the dorm that night, witnessing a pack of twenty-or-so Betsumon attacking everyone inside the Hazakura dormitory and stealing their identities.
On the next day, Hiro invites Gammamon and his impersonator to a picnic and dumps a pile of chocolate snacks for both of them. Gammamon eats all the chocolate quickly, but the Betsumon cannot eat too much of it, snapping Hiro out from the Betsumon's cognition-warping powers. The whole pack approaches Hiro and attacks him, and Gammamon digivolves into WezenGammamon to retaliate. The Betsumon are still Ultimate Digimon, so WezenGammamon's Sedna doesn't affect them, and they send him skywards with Tsukkomi Punch. However, WezenGammamon digivolves into Canoweissmon and defeats all of them in one fell swoop. All of them were detained by Clockmon afterwards.
When Kiyoshiro was in a coma because of being hit by a poltergeist-controlled truck from Sepikmon, the nurse who sends him into the hospital reveals itself to be a Betsumon ordered by Clockmon to bring Kiyoshiro to Mummymon. He makes it clear that he's only helping Kiyoshiro under Clockmon's orders.
(Source: Digimon Wiki)