Jimmy takes a journey to a far-off comet called Twonkus 3 and some adorable alien creatures called Twonkies follow him back to Earth in his sand samples. Carl gives them away as pets, despite Jimmy's warnings. When Jimmy finds out the Twonkies become dangerous monsters when exposed to music, he and his friends must round them up and send them back to space before it's too late.
| Actor | Role |
|---|---|
 Debi DerryberryJimmy Neutron (voice) | Jimmy Neutron (voice) |
 Mark DeCarloHugh Neutron (voice) | Hugh Neutron (voice) |
 Megan CavanaghJudy / Vox 2000 (voice) | Judy / Vox 2000 (voice) |
 Rob PaulsenCarl / Mr. Wheezer / Butch (voice) | Carl / Mr. Wheezer / Butch (voice) |
 Jeffrey GarciaSheen (voice) (as Jeff Garcia) | Sheen (voice) (as Jeff Garcia) |
 Carolyn LawrenceCindy Vortex (voice) | Cindy Vortex (voice) |
 Crystal ScalesLibby (voice) | Libby (voice) |
 Andrea MartinMs. Fowl (voice) | Ms. Fowl (voice) |
 Billy WestSam / Louie (voice) | Sam / Louie (voice) |
 Candi MiloNick (voice) | Nick (voice) |
| JJeffrey M. GunnNick's Singing Voice (as Jeff Gun) | Nick's Singing Voice (as Jeff Gun) |
 Frank WelkerGoddard / Twonkies (voice) | Goddard / Twonkies (voice) |
| GGraystarThemselves (voice) | Themselves (voice) |