Children all over London are disappearing after seeing visions of a clown holding a red balloon. Clyde and new student Rani are also getting visions of this clown. Those who see the clown were given tickets to a museum of circuses. Sarah Jane and Clyde visit this museum where they meet Elijah Spellman, who commands an army of mechanical clowns and can shapeshift into Odd Bob, the clown who has been abducting children. While there, they learn that Spellman was the inspiration for the legend of the Pied Piper, who abducted all the children in a village.
Scanning one of the tickets, Mr. Smith finds that Spellman comes from a nebula populated by creatures made of emotion. He also finds that the historical village in which the Pied Piper story takes place was the crash site for a meteor from the same nebula, now housed at the PHAROS Institute. Sarah Jane goes to the Institute and obtains a sample of the meteor.
Spellman feeds off the emotion of fear. He abducts children in order to produce fear in their parents. He uses his power to lure all the children from Luke's school to the museum but Sarah Jane and Mr. Smith use cell phone signals to interfere with his powers, freeing the children. However, Spellman is able to capture Luke.
Sarah Jane confronts Spellman, who feeds off her childhood fear of clowns. Rani realizes that the best way to fight fear is with laughter. Clyde tells several jokes, weakening Spellman and allowing Sarah Jane to suck him back into the meteor fragment. The children he captured, including Luke, are released.
This story marks the first appearance of Rani Chandra and her family.
- Adult Fear: Deliberately invoked by Spellman.
- Circus of Fear/Monster Clown
- Continuity Nod: When Sarah Jane is looking at pictures of clowns on her laptop, one of the pictures she pulls up is Clara the clown from The Celestial Toymaker.
- The PHAROS Institute previously appeared in The Lost Boy, and now owe Sarah Jane a favour for not exposing their part in the events of that story.
- Little Known Facts:Luke: Johnny Depp has [coulrophobia].Clyde: What encyclopedia did you read that in?Luke: Heat.
- Shout-Out: A good deal more than "99 Red Balloons" signify pending tragedy. The tragedy is averted.
- Why Did It Have to Be Snakes?: Sarah Jane has a childhood fear of clowns. Guess what she has to go up against.