Whenever I read a Kazuo Ishiguro novel, I have to prepare myself to be blanketed in a beautiful melancholy. "When We Were Orphans" is no different.
Like other Ishiguro novels, the protagonist circles around being honest about themselves and their emotions in such a way that the narrative similarly circles around the core of the plot. It gives the reading experience a mystery novel aura which Ishiguro leans into here, being having his protagonist BE an early 20th century private detective, bumping gently against the reader's awareness of Sherlock Holmes. His cases are regularly referenced, in dialogue and narrative, the part structure of the novel preparing you each time for a standard narrative, THIS is where the detecting will happen, only at the end to go okay, this was setting up the mystery, but now you can see this is akin to a cliffhanger and the NEXT part will be the mystery.
Christopher Banks is not a detective in a mystery novel here, but the way he thinks about things, the way he approaches and avoids elements of his life makes you certain he probably is, elsewhere. You're just not reading that book.
The central mystery of the novel and Christopher's life is the disappearance of his parents when he was a young boy living in Shanghai. It leads to his career path, which makes sense, and is something he is determined to solve, long into his adulthood, to the point you start going "Oh buddy", and when it becomes clear he expects to not only discover why they disappeared but to rescue them you go "Oh buddy, no, that's not how anything works".
Around the point in the book Christopher actually returns to Shanghai, the narrative goes from mystery novel shaped by adolescence expectations to something akin to Kafka's "The Trial", partially because Christopher has stepped into something twisting and elusive but partially because his denial keeps him from meeting certain things from a realistic, grounded perspective, from the man at the British consulate who is more interested in making plans for a party celebrating Christopher's success and rescue of his parents to his certainty that an injured Japanese soldier he finds in his search is his childhood friend from his time in Shanghai.
I'm sad at the end, but in a satisfied way. Ishiguro doesn't seem to write traditionally happy endings, but they're reflective, with a protagonist having, ideally, come to a better understanding of themselves, their past, and their feelings, and being able to have a realistic kindness for themselves going forward.