This book begins with a homeless teenager being warmly received by people at a shelter run by a cult. That teenager later reveals his name - Frank Hardy.
We then flash back to a few days earlier when the boys are awoken at 5 AM to discover that an old family friend, the banker Emmett Strand is talking to their father. The man is desperate, as his daughter Holly, who Frank knew as a younger kid, has run away and joined a cult. Fenton Hardy reveals that since Holly is eighteen, he cannot do anything to compel her to return. So, Frank, who once had feelings for Holly, plans to infiltrate the cult and bring her home.
Tropes found here are
- Crazy Survivalist: “Rosie”, who is hiding out in a cabin, knows some back roads to a bend in the Hudson River which also contains a sharp bend in railway tracks. He advises the boys and Holly to hop onto a freight train that must slow down to make this turn.
- Cult: A standard Scam Religion run by conmen who prey on rich or upper-middle-class kids to sign over their fortunes to the leader, the Rajah.
- Daddy Issues: Holly has them in spades. So does the Rajah, who turns out to have the same father as Holly.
- Deprogram: The denouement reveals that Holly has been going through a lot of therapy to shake off the Rajah's brainwashing of her, and is doing a lot better.
- Double Reverse Quadruple Agent: Holly, who is more brainwashed by the Rajah than anyone realized, allowed herself to be “rescued” as part of the Rajah’s plan to once again get close to her father and set him up for the Rajah’s vengeance.
- False Innocence Trick / Playing the Victim Card: Holly frequently does this to get the boys to stop asking her questions so they won't catch on that she's actually still loyal to the Rajah and secretly working towards accomplishing his plans.
- Frame-Up: The Rajah murders his Dragon, "Vivasvat", and claims to the local corrupt authorities that the Hardys were responsible.
- If You're So Evil, Eat This Kitten!: At his "induction ceremony" into the cult, Frank is renamed “Vaisravana” and is ordered to burn the captured Joe alive to prove his loyalty. Naturally, Frank frees Joe instead and the two manage to escape.
- The Infiltration: Frank pretends to be a homeless runaway to infiltrate the cult and rescue Holly. He abandons the charade to save Joe.
- Love Redeems: Frank admits the feelings he has for Holly to get her to stand down, and explaining to her that love comes in different forms.
- Luke, You Are My Father: The climax reveals that the Rajah is actually Paul Strand, Emmet Strand's biological son by his first wife. Paul wants revenge on Strand because he believed his father abandoned him and his mother, but it turns out that Strand's wife never told him she was pregnant when she left him, so Strand genuinely never knew his son existed.
- The One That Got Away: Holly seems to see Frank as this, wondering if they would have ended up together if he hadn't gotten with Callie Shaw. Frank is clearly attracted to her, too, but remains loyal to Callie.
- Pragmatic Villainy: The Dragon, "Vivasvat" (real name Shakey Leland), complains to the Rajah that the Revenge scheme the latter has concocted with Holly will only jeopardize the pretty good scam they have going for them right now. This pisses the Rajah off, who kills him and frames the Hardys for it.
- The Reveal: The Rajah is actually Emmet Strand’s son and Holly’s elder half-brother, Paul Michael Strand. This is the real reason he calls Holly "little sister" throughout the book.
