Theories

Plot
Ben and Rex try to figure out why Rex suddenly appeared in Bellwood

By Ben

 * Humungousaur (first re-appearance; accidental transformation, selected alien was Diamondhead)
 * Chromastone (first re-appearance; accidental transformation, selected alien was Cannonbolt)
 * Nanomech (first re-appearance)
 * Echo Echo (first re-appearance)
 * AmpFibian (first re-appearance)

By Khyber's Pet

 * Tyrannopede (first re-appearance)

Characters

 * Ben Tennyson
 * Rex Salazar
 * Rook Blonko
 * Max Tennyson
 * Magister Patelliday
 * Daughter Kineceleran (first re-appearance; cameo)
 * Citizens of Undertown
 * Ester (first re-appearance; cameo)

Villains

 * Hot Shot (first appearance)
 * Mallet (first appearance)
 * Friki (first appearance)
 * Khyber