Mysterions is an episode of the ALVINNN!!! and The Chipmunks series.
When Theodore discovers Alvin and Simon have a secret club, he promises not to tell anyone if they let him join.
- Alvin Seville
- Simon Seville
- Theodore Seville
- Dave Seville (cameo)
- Brittany Miller
- Jeanette Miller (cameo)
- Eleanor Miller
- Derek Smalls (cameo)
- Bocarter Humphrey (cameo)
- Kevin (cameo)
- Amber (cameo)
- Annie (cameo)
- Tommy (cameo)
- Bruce (cameo)
- "Out of the Light" 
- A sneak peek clip was posted on Nicktoons UK.
- The My Little Horsie club name is most likely a reference to Hasbro's My Little Pony franchise.
- The Orb makes a reappearance.
- When Theodore's practicing the secret handshake for himself, an instrumental of "Look at Me Now" plays shortly during the scene.
- The instrumental is then heard again later on, along with an instrumental of "Run the Runway."
- The mask that Brittany wears in Brittany The Body Snatcher makes a reappearance on the TV.
- Some of The Chipmunks' old outfits make a reappearance.