A mystery set in an urban setting, with modern young students being haunted by a vandal who's destroyed backpacks and lecture notes, and stolen things of no real value.
Poirot is flabbergasted when he discovers a couple of errors in the letter he just dictated to his secretary, Miss Lemon. It turns out that she is preoccupied because someone is committing several small crimes at the student home where her sister is the manager. Poirot's interest is awakened, and when he receives a list of the items that have disappeared or been destroyed, he is intrigued enough to secure an invitation to the student home.
This is probably Christie's first "modern" mystery, in an urban setting. Another few will follow - Third Girl, definitely, and The Clocks as well, and one could argue that while the setting is Miss Marple's village, The Mirror Crack'd From Side to Side also belongs to this category, with its descriptions of the village's development.
So, an urban Christie mystery. But is it any good? Well... It's not bad, per se. It's just that there's not a whole lot of detection in this story. Poirot turns up at the student home, makes a declaration, and one of the culprits confesses almost immediately.
Of course, this being Christie, there's more layers to the story, but hardly any of the real villains are found through fair play clues. Instead, they are just revealed in conversations during the course of the story.
And just why did they have to pour green ink over one of the student's notes? This was not done by the first vandal who confessed, so there was no need for that, plot-wise.
But it's still an interesting novel, with a plot that is just about as twisty as you'd expect from Christie, and the characters in the student house can be likened to one of Christie's country house families - there's almost the same type of characters and their interactions are also quite similar.
This is also notable as the first novel where Miss Lemon appears as Poirot's secretary, though she'd been in a number of short stories with Poirot before.
On the whole, a passable novel, though nowhere near Christie's heights. I'd rate this a 47 out of 100.