In this third and final installment of both live-action franchises of the films, Scooby and the gang have a high school reunion with Velma's science teacher, Icarus Young, who has a way of sending the gang back 17 years into the past where Fred and Shaggy were 17 year olds, Daphne was a 16-year old, Velma was a 15-year old and Scooby was a 7-year old, as they must solve the mystery of the Time Clock Ghost.
March 25, 2011
- Mystery Incorporated (Year 2003, the Present Day):
- Scooby-Doo (Neil Fanning) - Age: 24
- Fred Jones (Freddie Prinze, Jr.) - Age: 34, remembers he dyed his hair blonde because it looked good
- Daphne Blake (Sarah Michelle Gellar) - Age: 33, teaches her younger self Kung-fu
- Velma Dinkley (Linda Cardellini) - Age: 32
- Shaggy Rogers (Matthew Lillard) - Age: 34
- Mystery Incorporated (Year 1986, the Past):
- Scooby-Doo (Frank Welker) - Age: 7
- Fred Jones (Robbie Amell) - Age: 17
- Daphne Blake (Kate Melton) - Age: 16
- Velma Dinkley (Hayley Kiyoko) - Age: 15
- Shaggy Rogers (Nick Palatas) - Age: 17
- Paradox the Time Clock Ghost (Gary Oldman) - the main antagonist, his appearance is based on Ruber's, his goal is to claim Scooby for his Time Travel in 13 Ghosts of Scooby Doo.
- Professor Icarus Young (John Hannah) - Velma's old science teacher who sends her and the gang 17 years into the past to solve the mystery of the Time Clock Ghost, Reason: because of a certain clock found in his attic.
- Agent Porter Berkley (J.K. Simmons) - an FBI agent who warns the kids to stay off their backs, Reason: a file on Scooby was in his office when Fred was interviewed.
- Miles Cranston (Andy Serkis) - an old hermit who warns the gang of the Time Clock Ghost, Reason: he seams to have feared the legend.
- Mayor Hillary Lance (Emma Thompson) - the Mayor of Plymouth, a town in England, Reason: she is very tenacious when it comes to time.
- Orson Paxton (Peter Dinklage) - the owner of a Clock store, Reason: seems to have a symbol of the Time Ghost.
- Dr. Ferdinand Conrad (Gary Oldman) - He is the doctor who checks up on Scooby, Reason: he sounds like the ghost
- Clocks that are similar to the ghosts.
- Golden Cogs
- a schedule from 1984 on December 29th.
- Fred, Velma, Daphne, Shaggy, and Scooby's photographs.
- Agent Porter Berkley/Paradox the Time-Clock Ghost (J.K. Simmons/Gary Oldman) - Reason: to get revenge on Dr. Conrad for not saving his son on December 29th 1984, the year he died, so created to Ghost to manipulate Scooby and the gang to solve his son's death.