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Day of Doom is the sixth book and the final installment in the Cahills vs. Vespers series. It comes with cards 296-301. It was written by David Baldacci, a new author to this series, who has written books for adults like Zero Day, The Camel Club, and The Forgotten.


On the 39 Clues Message Board, Baldacci said:

"I can't promise that everyone, good or bad, makes it through to the end of Day of Doom. In fact, I can guarantee that many of you will be stunned by who doesn't make it. But the world of the 39 Clues is not for the faint-hearted. It's a dangerous world out there."


The Ultimate Sacrifice

It started with a kidnapping. A shadowy organization known only as the Vespers snatched seven members of the Cahill family and demanded a series of bizarre ransoms from around the world. Thirteen-year-old Dan Cahill and his older sister Amy began a global treasure hunt, determined to bring back whatever Vesper One needed, so long as it kept the hostages safe.


But when they deliver the last ransom, Amy and Dan discover Vesper One's terrifying endgame. The objects he demanded are vital pieces in a Vesper plot that will harm billions of innocent people. Now the two siblings and their friends are in an all-out sprint to stop Vesper One... before the whole world goes BOOM.


Starting from the last chapter of Trust No One, Dan Cahill drinks the "serum" that he made with the help of an Ekaterina scientist. He notices that nothing is happening, and Amy appears, stepping out of the shadows. She apologizes for running off when it seems that the Vespers had won, and tells Dan she couldn't let him do it. Somehow, Amy found out that Dan was creating the serum, and replaced the real serum with a mixture of vegetables. Dan gets mad, saying he was willing to die to save the world. Eventually Amy tells Dan that Isabel Kabra is Vesper Two, not Vesper One, and that they hacked into her flight plans and found out that Isabel is flying to Washington, D.C. Amy calls a meeting when Atticus and Jake Rosenbloom enter the room.

After a discussion, Atticus finally remembers that his mother had mentioned "Lewis and Clark" when she died. As the largest collection of Lewis and Clark items are in the National Museum of Natural History, Amy and Dan head to D.C. While on the train, Vesper One sends them a video of the hostages. Atticus realizes that Ted is blinking Morse code, and deciphers it. Ted was blinking: Riley McGrath is Vesper One. At the museum, they find out that Isabel had wanted to see the compass of Lewis and Clark. They meet a nice Lewis and Clark expert named Dr. Nancy Gwinn. She takes the four to see the Lewis and Clark compass. Upon examining it, Dan realizes there are numbers and letters scratched onto the back. When they go out of the museum, Isabel and her men arrived. They split up and Amy chases after Isabel. Isabel tricks her and she was nearly shot, but Dan saved her.

Meanwhile, Evan Tolliver and Ian Kabra get a call from Phoenix Wizard from a motel in Washington State and head there with Hamilton Holt and Jonah Wizard, who are followed by Sandy Bancroft, Casper Wyoming, and Cheyenne Wyoming. While eating, Cheyenne walks up disguised as a waitress and forces them into the back of the stolen Vesper van, taking them to the Rocky Mountains. There, Ian gets taken out of the truck by Casper, who takes him to see his mom, by video chat. Isabel Kabra convinces Ian to kill one of the Hostages by saying she is "poisoned by the hospital attendant" and dying. Ian fakes believing her and while meeting his sister as a favor from his mom, Vesper One asks what is Ian doing here. Ian feeds him some lies about Isabel, Sandy, Casper and Cheyenne plotting against him. Vesper One believes him, and as a "favor," gives Ian a choice to kill either Fiske Cahill, Ted Starling or Natalie Kabra. Ian stalls for time, saying that he hasn't made up his mind yet.

Dan decides to take a train to the Cascade Mountain Range, where they think the hostages and the Machina Fini Mundi are. At the station, Isabel makes an announcement, stating that the apocalypse is coming and that AWW will help. While hiding from her, Amy and Jake made their feelings clear to each other, and they kiss. During the night, Atticus grows suspicious of Dave Speminer, who was in the perfect position to kill his mother. He finds out that Dave Speminer is an anagram of Damien Vesper. He decides to break into Isabel's compartment to search for clues. Dan finds out that Isabel had been posing as his father and sending the text messages. Isabel and her men ambush her, but Sinead Starling saves her. Unfortunately, Atticus is again kidnapped and Isabel takes the serum, which was actually still in Dan's bag. Then Isabel stops the train and escapes. Dan has an idea, wearing the night vision goggles he got from one of Isabel's men and finding out that Isabel had messed the coordinates on the compass. The location of the hostages and the Machina Fini Mundi turn out to be at the Rocky Mountains, exactly where the train stopped at that point.

Meanwhile, the hostages and Ian Kabra, Evan Tolliver, Jonah Wizard, and Hamilton Holt, are being moved from the Cascades, to the Rocky Mountains, where they break free and beat up Sandy Bancroft, Casper Wyoming, and Cheyenne Wyoming. They meet Amy, Dan, Atticus and Jake. They try to find the Machina Fini Mundi and eventually found it. They start fighting with the Vespers. Evan gets shot and dies, while Natalie tries to destroy the device, but instead gets electrocuted and passes out. The remaining Cahills decide to protect the Machina Fini Mundi from Vesper One. Through his super secure phone, Vesper One instructs his men to kill Isabel Kabra.

Meanwhile, Isabel is wandering the maze alone, as her guards had been scared off by Atticus's iPod. She passes by a room with monitors and sees Ian crying and Natalie's dead body. Isabel, wanting revenge, drinks the serum.

Amy manages to turn the Machina Fini Mundi into a giant electromagnet. When the Vespers come, their guns, swords and knives got attracted to the magnet and they get electrocuted. Sandy and Vesper One arrive and hand out stone weapons. Just at that time, Isabel comes in and tries to stop Damien from putting the final piece but fails. The device gets powered but Isabel starts ripping it apart. Damien melts into the device, ruining it and it explodes, killing Damien and Isabel. A falling rock hits Amy's head. She has a dream vision about the people who had died, and was shocked to see that Isabel and Natalie seemed to be happy even though they are dead. Then she awakes and finds out that Jake had carried her out, and that she had been unconscious for seven days. The story ends off with Amy and Dan discussing about how they would return to their normal lives again.

In the audio-book, there is an audio extra about a TV show about Amy and Dan's "stealing" of the Carravagio Medusa.


The book puzzle key is different weather symbols. It reads: One Other Has The Serum. This is referring to J. Rutherford Pierce and the events of UNSTOPPABLE series.

On the cover of the book, it says "DATE SHIFT: 9/18/1793 WBBBSRH". Each letter is that many places backward in each number of the date. When decoded, it says "NATALIE."


  • In Into the Gauntlet, the logo shatters into pieces. But on the cover of this book, it is tampered with again: it gets struck by lightning and burns up and turns into ashes.
  • This book is notable for having the most confirmed deaths in the series with a total of four: Natalie Kabra, Evan Tolliver, Damien Vesper and Isabel Kabra.
  • Day of Doom's cards show together a map of all of the Vesper bases and some of them are already shown in the books, such as Czech Republic (Prague) in A King's Ransom and India (Mumbai) in Shatterproof.


In every last book of a series, a front cover shows a destroyed 39 Clues Logo.


Capitol Logo


The cards 296-301 form a Vesper world map:


Day of Doom trailer[]