Holmes and Watson in “Unfinished Business”

It's obvious what happened: one of the original team sold the recipe for breaching The Leviathan to an outside party.

Watson

That's obvious?

When you've eliminated the impossible, whatever remains, however improbable, is the truth.

Watson

And what's impossible here is that you couldn't break into the safe but someone else could. In other words, you are the smartest man in the world. [pause] I'm waiting for you to challenge me and say that's ridiculous.