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"The Yeerks have managed to repair the Helmacron ship and they've reactivated the sensors that locate morphing energy. [...] That would be the energy from the morphing cube. [...] That's why you and the others have to keep it moving until the Helmacron ship can be destroyed."
Erek King to Cassie[src]

The Hidden, published in February 2000 and written by Laura Battyanyi-Wiess based on an outline by K.A. Applegate and Michael Grant, is the thirty-ninth book in the Animorphs series. It is the eleventh book narrated by Cassie and her sixth book as sole narrator.


Official Description

The Yeerks have discovered and repaired a damaged Helmacron ship. They know of its morph-seeking capabilities, and they plan to use the ship to capture the "Andalite bandits." And to find Elfangor's blue cube. The one that gave the kids the ability to morph.

Cassie, the other Animorphs, and Ax are in a pretty bad situation because they can't leave the cube in one place, they can't morph without being discovered, and they have to keep moving. It looks like this may be a battle the Visser can't lose...

Plot Summary

Animorphs 39 The Hidden inside cover only

Cassie in buffalo morph

The Yeerks repair a downed Helmacron ship and use its sensors to track the Escafil Device and the Animorphs morphing abilities. Cassie is forced to relocate the Escafil Device. During the process, a Cape buffalo and an ant inadvertently gain the morphing ability. The buffalo morphs Chapman and begins to learn speech, and the ant morphs Cassie. Cassie kills the ant when it demorphs, and near the end of the book the buffalo is killed by a Dracon beam.



Supporting Characters:


Other Characters:



Major/Highlighted Events


Morpher Morphs acquired Morphs used
Jake -- Seagull, Peregrine Falcon (possibly), Great Horned Owl (possibly), Wolf, Siberian Tiger, Housefly, Bottlenose Dolphin
Rachel -- Seagull, Great Horned Owl, Wolf, Grizzly Bear, Housefly, Bottlenose Dolphin
Tobias -- Housefly, Bottlenose Dolphin
Cassie African Cape Buffalo (Buffa-Human) African Cape Buffalo (Buffa-Human), Wolf, Housefly, Osprey, Humpback Whale (partially)
Marco -- Seagull, Osprey (possibly), Great Horned Owl (possibly), Wolf, Silverback Gorilla (Big Jim), Housefly, Bottlenose Dolphin
Ax -- Seagull, Northern Harrier (possibly), Great Horned Owl (possibly), Housefly, Bottlenose Dolphin
Visser Three -- Human (Main), Unnamed Acidic Tentacled Creature
Buffa-Human Human (Hedrick Chapman), Andalite (Alloran-Semitur-Corrass) Human (Hedrick Chapman), Andalite (Alloran-Semitur-Corrass)
Ant-Cassie Human (Cassie) Human (Cassie; partially)


  • The front cover quote is, "To morph or not to morph? That is the question...."
  • The inside front cover quote is, "Where the buffalo roam..."
  • The inside cover illustration shows the police officers' patch as saying "ORSON POLICE." Orson is the name of cover artist David Mattingly's pet cat, and he has shown or spelled Orson on previous inside cover illustratons before.
  • This marks the tenth time that an Animorph is acquired, the second time that Cassie is acquired, and the second time that Alloran-Semitur-Corrass is acquired and morphed.
    • This is also the first instance where an Animorph is acquired and morphed by a non-Animorph.
  • This marks the fifth time that the Chee pose as the Animorphs while they are on a mission.
  • Cassie makes a reference to the events of The Andalite's Gift; she mentions that their mission of having the others in dolphin morph lure a morphing energy sensor into the ocean as she attempts to destroy it as a humpback whale was something they had done before.


  • Cassie claims that the Yeerks are like a disease filled with malice and intent, despite the fact that at this point, she, of all the Animorphs, is aware that the Yeerks are not collectively filled with malicious intent.
  • Cassie states that Elfangor gave them the blue box prior to being murdered by Visser Three; Elfangor did no such thing. The box was presumed destroyed upon Elfangor's death until being found by David, at which point the Animorphs recovered it.
  • The sensor from the Helmacron ship, which the Yeerks salvaged from The Suspicion, is able to track the energy put off by the blue box and the morphing energy signature emitted by the Animorphs while morphing or demorphing. However, in The Suspicion, the Helmacrons revealed that their sensors also could tell which individuals were morph-capable even when in their normal forms, as they still emitted a signature. Since the Yeerks were using the same sensor, the Yeerks should have been able to track the Animorphs even when they were not morphing or in possession of the blue box.
  • There is an extra space when Jake says <Cassie, move,> on p12.
  • An African Cape Buffalo acquires the power to morph by brushing past the blue box, and later an ant is able to gain the same power by crawling on the box. However, based on the rules of morphing provided thus far, one can only acquire the power to morph by maintaining direct, physical contact with the box while a morph-capable user activates it. This is why David, despite possessing and manhandling the box, did not receive the power until it was activated by Ax.
    • The buffalo is able to acquire Hedrick Chapman and Alloran-Semitur-Corrass after brief contact. However, the series thus far has established that one must maintain physical contact for a few seconds while simultaneously using focus and concentration in order to acquire and subsequently morph into another creature. The ant also would not be able to acquire or morph Cassie since the series also established that ants do not possess a mind of their own, let alone one that is capable of focus, concentration or awareness of any kind.
      • The ant is able to acquire Cassie without placing Cassie in a trance. While it is possible for some to be immune to the trance, Cassie is not, as she fell under the acquisition trance in The Message.
  • It is alluded that Ax is running with the others in wolf morph while it is also stated that Tobias is "running" and later "demorphed", although neither Ax nor Tobias possess wolf morphs.
  • Cassie's line is written as if it was said in thought-speak on p93, despite the fact that she is in human form.


U.S. Images

International Covers

Main Series #1 The Invasion | #2 The Visitor | #3 The Encounter | #4 The Message | #5 The Predator | #6 The Capture | #7 The Stranger | #8 The Alien | #9 The Secret | #10 The Android | #11 The Forgotten | #12 The Reaction | #13 The Change | #14 The Unknown | #15 The Escape | #16 The Warning | #17 The Underground | #18 The Decision | #19 The Departure | #20 The Discovery | #21 The Threat | #22 The Solution | #23 The Pretender | #24 The Suspicion | #25 The Extreme | #26 The Attack | #27 The Exposed | #28 The Experiment | #29 The Sickness | #30 The Reunion | #31 The Conspiracy | #32 The Separation | #33 The Illusion | #34 The Prophecy | #35 The Proposal | #36 The Mutation | #37 The Weakness | #38 The Arrival | #39 The Hidden | #40 The Other | #41 The Familiar | #42 The Journey | #43 The Test | #44 The Unexpected | #45 The Revelation | #46 The Deception | #47 The Resistance | #48 The Return | #49 The Diversion | #50 The Ultimate | #51 The Absolute | #52 The Sacrifice | #53 The Answer | #54 The Beginning
Megamorphs The Andalite's Gift | In the Time of Dinosaurs | Elfangor's Secret | Back to Before
Chronicles The Andalite Chronicles | The Hork-Bajir Chronicles | VISSER | The Ellimist Chronicles
Alternamorphs The First Journey | The Next Passage
TV Tie-Ins Meet the Stars of Animorphs
Graphic Novels #1 The Invasion | #2 The Visitor | #3 The Encounter | #4 The Message | #5 The Predator | #6 The Capture