The premise is sraightforward enough. During his service in Afghanistan, Watson was called upon to "carry letters" and basically serve as a spy on behalf of queen and country. A relic arrives in the mail warning him of his danger, and he and Holmes have to move to save Watson's life and good name. Wonderful atmosphere, incredible use of suspense, and the touches of Indian culture give it a nicely exotic feel (without engaging in stereotypes too much for the period, I didn't think). It's Doyle canon, but if you're only familiar with the BBC it's really still very enjoyable without knowledge of specific Doyle stories. Though a general sense of the kind of characters Holmes and Watson are would be helpful.
Do check it out.