N or M?, Pt. 3

S1 E6: Tommy has finally found the missing scientist, Gilbert Worthing, but they are both being held against their will at Commander Haydock’s, and we’re still none the wiser as to who N really is. It turns out that Gilbert managed to hide a vital key to his device before his capture, and that without it the bomb N and Haydock have stolen is entirely useless. So when Tuppence storms Haydock’s house and helps Tommy and Gilbert to escape, it seems like the only thing left to do is retrieve the key from the strange place Gilbert hid it. The only problem is that it’s in the Sans Souci, where N is lying in wait. When N gains possession of the key Tommy and Tuppence find themselves once again backed into a corner. With Carter’s help they must come up with their most ambitious and risky plan yet to save the lives of thousands of innocent British citizens.