|Non-Canon: Characters and/or events depicted in this article are non-canon to the main continuity.|
An attempt to catch a thief who's stealing tech from the inhabitants of Undertown puts Ben and Rook face-to-face with someone from their past whose very existence could mean the end of the world as we know it. To save the universe (again), Ben will have to make one of the hardest decisions of his life. Can he do it before the parallel paradox unravels the very thread of history itself?!
- Ben Tennyson (16 years old)
- Ben Tennyson (11 years old, flashback)
- Gwen Tennyson (11 years old, flashback)
- Kevin Levin (cameo, flashback)
- Julie Yamamoto (cameo, flashback)
- Elena Validus (11 years old, flashback)
- Cash Murray (flashback)
- J.T. (flashback)
- Marleys (flashback)
- Jane Marley (flashback)
- Rook Blonko
- Professor Paradox
- Dr. Animo
- Vreedle Brothers
- Elena Validus (16 years old)
- Victor Validus (cameo, flashback)
Alternate Timeline Characters
- Cube Helmet
- Mrs. Baumann
- Elena Validus
- Max Tennyson
16 year old Ben
- Way Big (cameo, flashback)
- Ultimate Echo Echo (cameo, flashback)
11 year old Ben
- 11 year old Snare-oh has his 16 year old appearance.
- The Splixson Plumber recruit trainee Plen Stroff has an Omnitrix symbol on his forehead.
- In her first appearance, the alternate timeline Fistina has her normal timeline color scheme.