Harry Potter fans still ask who rats out Dumbledore’s Army because the book and movie give two different answers.
In Harry Potter and the Order of the Phoenix, the betrayal comes from Marietta Edgecombe, while the film shifts that role to Cho Chang.
In the book, Marietta informs on the group.
In the movie, Cho carries that plot point instead.
That change makes the answer confusing, especially if you remember the scenes from Hogwarts, the Ministry of Magic pressure, and Umbridge’s crackdown on student resistance.
The Short Answer In The Books And Movies
The answer depends on which version you mean.
In the book, Marietta Edgecombe betrays the group.
In the film, Cho Chang is the one linked to the leak.
Marietta Edgecombe Betrays The D.A. In The Book
In Harry Potter and the Order of the Phoenix, Marietta tells Dolores Umbridge about Dumbledore’s Army.
The book presents her as a Ravenclaw student who joins reluctantly, then panics because her mother works for the Ministry of Magic and fears losing her job.
Hermione’s enchanted sign-up sheet makes the betrayal unforgettable.
Marietta ends up with the word “SNEAK” across her face.
That punishment is part of why book readers usually remember her name first.
Cho Chang Takes The Fall In The Film
The movie removes Marietta’s role and gives the betrayal to Cho Chang.
That choice simplifies the plot, especially because the film has less room to explain the full student network and the Ministry’s pressure campaign.
It also changes how you read Cho’s relationship with Harry Potter and the rest of the group.
The film uses her as a shortcut for tension, even though the book keeps the blame on her friend.
Why Fans Still Mix Up The Two Versions
Fans mix them up because both versions center on the same exposed secret group, and both connect the leak to Umbridge’s punishment of students.
Since Cho and Marietta are both tied to Ravenclaw and the D.A., the details blur fast.
The mix-up sticks because the movie trims supporting plot lines, while the book spreads the blame, fear, and fallout across more characters.
If you remember the emotion but not the names, the two stories can feel almost identical.
Why The Secret Group Existed In The First Place
Dumbledore’s Army formed as a response to fear, bad teaching, and the Ministry’s interference at Hogwarts.
You can trace it directly to the moment students realized they were not being prepared to defend themselves.
Dolores Umbridge And Theory-Only Lessons
During Harry Potter’s fifth year, Dolores Umbridge taught only textbook theory in Defense Against the Dark Arts.
That left students without practical defensive spells, duelling practice, or any real way to defend themselves against the dark arts.
Cornelius Fudge and the Ministry of Magic backed that approach because they wanted control and refused to accept Lord Voldemort’s return.
The classroom looked official but gave students almost nothing useful.
How Hermione, Harry, And Ron Built The Group
Hermione Granger first pushed the idea of a practical study group.
Harry Potter agreed to teach it.
Ron Weasley supported the plan because the three of them knew that students needed real training to survive what was coming.
The group started small, with fifth-year students leading the way and word spreading quietly through Hogwarts.
What began as exam prep quickly became serious resistance training for the wizarding world.
The Hog’s Head Meeting And The Room of Requirement
Hermione arranged the first meeting at the Hog’s Head in Hogsmeade, where students gathered under awkward, tense conditions.
After that, the group moved into the Room of Requirement, which gave them a hidden place to practice.
That setting mattered because it let the D.A. grow without drawing immediate attention.
It also made the group feel secret, urgent, and very real to the students involved.
How The Betrayal Happened And What It Caused
The leak grew from fear, family pressure, and Umbridge’s surveillance.
Once the information reached the Ministry of Magic side of the school, the crackdown came quickly.
Marietta’s Motive And Her Mother’s Ministry Ties
In the book, Marietta Edgecombe feels pressure from her mother’s position at the Ministry and worries about the consequences of being caught.
That fear makes her vulnerable to Umbridge, especially with Cornelius Fudge backing the campaign against dissent.
Her choice fits the book’s wider pattern, where adults in power push students into impossible corners.
It is less about malice and more about panic under pressure.
Hermione’s Jinxed Sign-Up Sheet
Hermione Granger built protection into the sign-up parchment by jinxing it so a betrayer would be marked with “SNEAK” on their face.
That detail is one of the most memorable parts of the storyline, and it confirms that the books wanted a clear culprit.
The jinx also shows how seriously Hermione treated secrecy.
She knew the group could not survive if someone handed it over to Umbridge.
Dumbledore’s Intervention And Umbridge’s Crackdown
Once the D.A. was exposed, Dolores Umbridge used the information to tighten control at Hogwarts.
Her actions escalated quickly.
Albus Dumbledore stepped in when the situation turned into a full confrontation.
Severus Snape later becomes part of the broader school power shift, while the betrayal itself remains tied to the D.A.’s early exposure.
Cedric Diggory’s death and the Ministry’s denial also hang over the moment, which is why the secrecy feels so politically charged.
Why The D.A. Still Mattered After Being Exposed
Even after the group was exposed, the training mattered because the students had already learned how to fight back.
The lessons kept echoing through Hogwarts long after the first meeting ended.
The Members Who Carried The Training Forward
Harry Potter, Ginny Weasley, Neville Longbottom, and Luna Lovegood helped keep the spirit of the D.A. alive.
Other members like Ron Weasley, Angelina Johnson, Dean Thomas, Lavender Brown, Lee Jordan, Ernie Macmillan, Hannah Abbott, Susan Bones, Michael Corner, and students from Hufflepuff and Ravenclaw carried the skills they learned into later resistance.
The group was never just about one secret meeting.
It was about building confidence, memory, and muscle for the battles ahead.
Resistance Under The Carrows
When Death Eaters took control of Hogwarts, Alecto Carrow and Amycus Carrow turned the school into a harsher place.
Students who learned from the D.A. were better prepared to resist, hide, and protect one another.
The threat of the Cruciatus Curse and other Dark Arts lessons made those earlier sessions feel even more important.
The D.A. proved that students could still organize even under brutal rule.
From Secret Lessons To The Battle of Hogwarts
By the time of the Battle of Hogwarts, the D.A. was no longer just a study group.
It had become a real network of students who acted against Lord Voldemort and the Death Eaters.
That arc runs through Harry Potter and the Order of the Phoenix and Half-Blood Prince.
It continues in Deathly Hallows.
The group started as students who practiced in secret in the Room of Requirement.
Those sessions helped shape the defenders who stood up in the final battle.
