I would agree with other reviewers on this site that The Mission Song is not one of Le Carre's best. In fact, comparing it to The Constant Gardener, it is almost hard to believe they are written by the same author. However, Le Carre on an off day is so much better than so many of his contemporaries that even a novel as flawed as this is worthy of four stars.
The Mission Song is narrated in the first person by Bruno Salvador - Salvo - a self confessed 'top interpreter' who is nevertheless naive and impressionable. Having just met the love of his life, he eagerly accepts a commission to translate for a shady conference that, he is told and he willingly believes, will save his native Congo. Of course, things are not what they seem.
So far, so Le Carre. Yes, the plot is predictable, but let's face it, most plots are. Given that the majority of the book is set in London or the nameless island where the conference takes place, I found The Mission Song lacked that transporting touch that characterises much of Le Carre's work. You may also find Salvo's somewhat florid speech and blind devotion to authority figures rather tiresome. But overall, The Mission Song is better than much of the dross that you'll find in the bookshops these days.