3 years after the finale of The Secret Saturdays, the Saturdays have been having family troubles from the past due to Zak's deal with Arghost, but they must work together again when a greater threat from space comes upon them, thanks to Arthur Beeman's revenge plot against Doc.
- The Secret Saturdays:
- Zak Saturday (Taylor Laughtner): now 16 years old and had been raised by Doyle ever since Zak's deal with Argost, he now goes to High school, and always harrasses bullies, has no bonding with his father
- Doc Saturday (Dwayne Johnson): has been having technical issues with his son, since Zak's deal with Argost, and had to send him away to be raised by Doyle
- Drew Saturday (Charlize Theron): has been upset at Doc's stubbornness to send Zak to live with Doyle, and that they never had Thanksgiving time together or Christmas time together
- Fiskerton (Dee Bradley Baker): Zak's best friend, and is disappointed in Doc for sending him away
- Zon (Dee Bradley Baker): Zak's pet, mad at Doc for sending him away
- Komodo (Dee Bradley Baker): Zak's other pet, also angry at Doc for sending him away
- Doyle Blackwell (Channing Tatum): Zak's uncle and Drew's brother, he had to raise Zak on his own after Zak's deal with Arghost
- Secret Scientists:
- Miranda Grey (Rachel Weisz): an old friend of Drew and uses a weapon to teleport
- Paul Cheechoo (Aziz Azari): specializes in cold climate research, he is mad at Doc for sending Zak away
- Wadi (Frieda Pinto): Zak's girlfriend, who has friction with Zak about his deal with Arghost
- Arthur Beeman (Guy Pearce): appears as the secondary antagonist, wanting revenge against Doc for firing him for nearly killing Zak, he uses the Horn of Heimdall to call upon a threat from space
- Scope-Dragon the threat from space and the main antagonist, it was called by Beeman in order to gain the planets natural resources.