’I admit it’s a steep proposition.  But after all how much ill can he do?  There are pretty strict limits to the activity of even the cleverest spy.’

’I agree.  But this man is not a spy who buys a few wretched subordinates and steals a dozen private letters.  He’s a genius who has been living as part of our English life.  There’s nothing he hasn’t seen.  He’s been on terms of intimacy with all kinds of politicians.  We know that.  He did it as Ivery.  They rather liked him, for he was clever and flattered them, and they told him things.  But God knows what he saw and heard in his other personalities.  For all I know he may have breakfasted at Downing Street with letters of introduction from President Wilson, or visited the Grand Fleet as a distinguished neutral.  Then think of the women; how they talk.  We’re the leakiest society on earth, and we safeguard ourselves by keeping dangerous people out of it.  We trust to our outer barrage.  But anyone who has really slipped inside has a million chances.  And this, remember, is one man in ten millions, a man whose brain never sleeps for a moment, who is quick to seize the slightest hint, who can piece a plan together out of a dozen bits of gossip.  It’s like—­it’s as if the Chief of the Intelligence Department were suddenly to desert to the enemy . . .  The ordinary spy knows only bits of unconnected facts.  This man knows our life and our way of thinking and everything about us.’

’Well, but a treatise on English life in time of war won’t do much good to the Boche.’

Sir Walter shook his head.  ’Don’t you realize the explosive stuff that is lying about?  Ivery knows enough to make the next German peace offensive really deadly—­not the blundering thing which it has been up to now, but something which gets our weak spots on the raw.  He knows enough to wreck our campaign in the field.  And the awful thing is that we don’t know just what he knows or what he is aiming for.  This war’s a packet of surprises.  Both sides are struggling for the margin, the little fraction of advantage, and between evenly matched enemies it’s just the extra atom of foreknowledge that tells.’

‘Then we’ve got to push off and get after him,’ I said cheerfully.

‘But what are you going to do?’ asked Macgillivray.  ’If it were merely a question of destroying an organization it might be managed, for an organization presents a big front.  But it’s a question of destroying this one man, and his front is a razor edge.  How are you going to find him?  It’s like looking for a needle in a haystack, and such a needle!  A needle which can become a piece of straw or a tin-tack when it chooses!’

‘All the same we’ve got to do it,’ I said, remembering old Peter’s lesson on fortitude, though I can’t say I was feeling very stout-hearted.