Cassidy Williams forms an uneasy alliance with Scooby and the gang after her radio station is destroyed by German Kriegstaufen robots. Their lead as to their originations lead them to an underwater city in a trench near Crystal Cove. To get there, Daphne recruits the two boys she used to babysit—Tom and Tubb from the 1967 Moby Dick And Mighty Mightor
cartoon show, along with their pet seal Scooby.
The episode provides examples of:
- Call Back: Angel had previously used CDs as an offensive weapon in "Revenge of the Man Crab".
- Crossover: Much like "Mystery Solvers Club Finals" the gang meets with Tom and Tubb from Moby Dick And Mighty Mightor. Their parents are revealed to be Dr. Paul Williams and Captain Michael Murphy from Sealab 2020.
- Dead All Along: The gang discovers Abigail Gluck's decayed corpse, even though the programming is still running. However, the word "Nibiru" is whispered from Gluck's lips.
- Downer Ending: Professor Pericles makes off with piece no. 6 of the Planispheric Disc, and it is implied that Cassidy did not survive the detonation of the city instigated by Pericles.
- Dying Moment of Awesome / Heroic Sacrifice / Killed Off for Real: Cassidy Williams died saving Mystery Incorporated from the blast. She does not return by the end of the series, which seems to confirm her death.
- She does return in the finale, after the Nibiru entity's influence is erased.
- Name's the Same: Scooby the seal meets Scooby-Doo.
- Never Say "Die": Averted. Not only do we see skeletons of the dead missionaries, but also a decaying corpse.
- A straighter example occurs when Fred says he's sure that Cassidy made it out.
- No Indoor Voice: Tom and Tubb, due to "water in the ears".
- No Swastikas: The "World War II Era Robots" as everyone calls them, resemble German stormtroopers, speak in broken German, and have red symbols that resemble the "no smoking" sign in place of swastikas on their helmets.
- Official Couple: It looks as if Mayor Nettles and Sheriff Stone are headed that way.
- Title Drop: Tom and Tubb explain that "The Midnight Zone" is an area so far down that there is no light.
- You Meddling Kids: Pericles actually uses the original phrase.