|“|| "Good-bye, Mac. Sorry about the time I put a frog down your pants. Thanks for taking care of us.”
— Dan Cahill in Nowhere to Run.
The information included in this article appears thoughout the entire 39 Clues universe.
Dan's parents, Hope Cahill and Arthur Trent, perished when he was young, and placed in the custody of his great-aunt Beatrice with his sister Amy. The children were watched by a revolving door of Au-pairs hired by Beatrice, and would frequently visit their grandmother Grace at her mansion. As a child, Dan became obsessed with ninjas and would frequently get into trouble for his mischievous behavior. On Christmas 2007, Dan narrowly avoided a kidnapping attempt by the Vespers.
When his grandmother, Grace Cahill, dies, Dan finds out he is part of the most powerful family on Earth, the Cahills. He sells his baseball cards and collections for about $3,000 to help fund the Clue Hunt. Dan is frequently called "dweeb" and "idiot" by Amy in this book, The Maze of Bones.
He and Amy are to find out the next clue by solving the sheet music of W.A.M. (Wolfgang Amadeus Mozart). Thanks to his photographic memory, he could re-create the lead they had early in the book that was destroyed accidentally by Saladin though they are tailed by their Tomas competitors, the Holt Family .
Dan and Amy act as the player agent's handlers, issuing instructions at the beginning of the mission.
Dan and Amy almost get kidnapped. They receive the Vesper phone. They have to steal Caravaggio's Medusa
Amy and Dan go to Switzerland to find the De Virga Mappa Mundi. Dan finds his father is alive. Atticus gets kidnapped.
Amy and Dan find Atticus and go to Uzbekistan in search of a stale orb, which turns out to mean astrolabe.
In Trust No One, Dan becomes the new leader of the whole operation, since Amy has given up. He is determined to not let down the hostages nor Atticus, and in the end he drinks "Gideon's master serum".
We first see Dan at the very beginning of the book, who just drunk "the serum". Amy appears a page or two later, explaining what she done with the serum and apologizing for running away when it seemed like the Vespers had won. Dan finds out that Isabel Kabra is Vesper Two, not Vesper One. He travels by train to the Machina Fini Mundi with Amy, Jake and Atticus, though Isabel Kabra was onboard. He finds out that Isabel Kabra was sending the "AJT" messages. He fights with the Vespers where the Machina Fini Mundi was and he witnesses the death of Dave and Isabel.
In Worlds Collide, Dan, the Cahills, and Hystorians, and Greencloaks are partying when J. Rutherford Pierce, Zerif, and Tilda burst in to steal talismans and the Infinity Ring. They are stopped, and after, a dance battle ensues between Dan, Briggan, and Christopher Columbus, in which Dan wins.
Dan and Amy travel to find the ingredients to the Antidote to the Master Serum.
They fight to stop Pierce from declaring his Presidential candidacy and start his diabolical plan to take over the world.
Amy and Dan are retired, and have handed over the reins of the Cahill Family to Ian, when the Outcast, a former Cahill, strikes with his plan for revenge. Amy and Dan have to stop him from recreating the Titanic disaster.
Amy and Dan try to stop the Outcast from recreating the Hindenburg disaster, while they realize the Outcast's true plan. They also find out that Nathaniel Hartford, their grandfather, is still alive.
Dan and the others have to stop the Outcast from recreating the disaster Hurricane Katrina. He and Amy are handed back the leadership of the Cahills.
Amy and Dan stop the Outcast from recreating the Chernobyl disaster. They defeat their grandfather and dispose of his deadly bees.
Dan is rather annoying to Amy because he is childish, but he cares deeply for his sister inside because she is the only loving family he has left. Dan loves to eat junk food and is obsessed with ninjas, foreign snacks, burp and fart jokes, and anything to do with video games. Dan and Amy get into plenty of fights and arguments, but they always forgive each other afterwards because they only have each other in the world. Even though Amy calls him "dweeb," for her that is like saying, "You're still my brother" to him. Dan shows love for Saladin even when he continuously scratches him in the first book. He also has photographic memory.
Dan has dark blond hair, a light complexion, and the same jade green eyes as Amy. He also has a small scar right under his right eye that he obtained from falling on his plastic AK-47 when he had gone "commando-raiding" at age seven. It is frequently hinted that he is a bit small for his age. He looks a lot like his father. He has a good sense of humor and didn’t really have any friends until he met Atticus.
He knows Morse code. He has a photographic memory. He is very good at math, a talent his dad may have passed on to him (according to the Maze of Bones). He is a natural at puzzle solving. He is good at figuring out mechanical things, like lock mechanisms.
Clues Known To Have
Dan has all 39 Clues, due to each competitor trusting Amy and Dan with their clues. Dan memorized it all, meaning the clues are safely in his head.
- He likes to collect items of all sorts, including a baseball collection worth more than $3,000, autographs of famous outlaws, Civil War weapons, rare coins, his casts from broken limbs (apparently 12 since preschool), charcoal and wax tombstone rubbings, and samurai swords.
- His favorite Tombstone Rubbing is that of Pruella Goode, which reads 'I'm Dead. Let's have a Party.'
- His middle name is Arthur, after his father.
- He is fascinated with ninjas, but he didn't like being in the ninja fight in The Sword Thief.
- On the 39 Clues website, on the Cahill web, Dan is revealed to have stolen the heads of mascot costumes and even once glued a teacher’s desk to the ceiling during recess. How he glued it to the ceiling is unknown, especially in such a short time and without an adult there to help him.
- He attended Adams Middle School.
- He apparently still wears Pokémon underwear, as stated in The Black Book of Buried Secrets and it said in another book that he still wears Superman pajamas.
- He got in trouble three times on his first day of 7th grade, all resulting in him being sent to the principal's office, from the Black Book of Buried Secrets: 1.) Disrupted class, threatened teacher 2.) Disrupted class, talked back to teacher. 3.) Disrupted class, endangered students.
- He has asthma, and will often forget his inhaler or have Amy and/or Nellie hold it for him.
- Dan often uses "ninja moves" in school.
- He asked if he could do his history report on Benjamin Franklin's interest in farts. The teacher scolded Dan for disrespecting one of the Founding Fathers. Dan then replied that Franklin's greatest accomplishment could not be ignored by anyone.
- The teacher asked why Dan had missed the first three weeks of school, and he replied that "It's top-secret. If I told you, my family might have to kill you."
- He also said that he was a ninja lord, and demonstrated by back-kicking and accidentally hitting the fishbowl. Herbert the goldfish was said to be in "critical condition".
- He brought Mozart's tungsten samurai sword in music class and waved it in the air, causing it to be confiscated.
- On several occasions, he seems to think to use the Master Serum but Amy always stops him from doing it. However, Dan has been collecting the Clues as a "last resort" against the Vespers. However, Amy takes the serum herself in Countdown to save Dan from falling to his death.
- Later on in the series, he becomes more responsible and starts to carry his inhaler with him, other than Nellie or Amy carrying it for him.
- It was said in the website as well as the back of Card 2 that he wore his ninja costume to school on November 4, resulting in a detention.
- Vesper Two believes that Dan can be turned, even though Vesper One disagrees.