"The Commodore has asked me to select a certain number of officers for a particular service," began the Commander. "It occurred to me that for various reasons you would be a suitable candidate. It is, of course, optional whether you accept or otherwise, since it is a matter requiring great discretion and involving a certain amount of risk, not to say danger." The "Bloke" paused and fixed his eyes upon the young officer. "Near East, for a dead cert," thought Cavendish, then aloud he said, "I'm quite ready, sir." "You'd better wait until you've learnt more of the nature of the operations," resumed the Commander, with a wry smile. "Let me see; you served a commission in the South American station, I believe?" "Yes, sir; midshipman on the _Cyclex_ in 1921-2." "You know the approaches to Bahia? And San Luiz? And Macapa? Good. Now, describe the anchorage off Port of Spain." "Weeds" did so, evidently to the Commander's satisfaction. "Do you know anything of the Rio Guaya?" continued his inquirer. "No, sir," replied Cavendish promptly. "We never put in there during the whole of the commission. But----" He paused, thinking that what he was about to say was irrelevant. "But what?" "I know a fellow living out in Rioguay, sir. An old shipmate of mine. He went on the beach from the _Baffin_." "Name?" "Peter Corbold, sir." "H'm; name's familiar. Do you ever hear from him?" "I had one letter, sir. I answered it--but I haven't heard since." "What's he doing out there?" "Mining engineering, I think, sir. He mentioned an uncle in the same profession who had been in Rioguay for some time." The Commander started on another tack. "The Admiralty have issued orders for the _Cynesephon_ to be brought forward for commissioning," he announced. Cavendish sat bolt upright in the chair. Now he was beginning to grasp the drift of things. Hitherto, he had been groping blindly, trying to piece together the baffling questions which the Commander had put to him, in a vain endeavour to discover the nature of the hazardous duty hinted at. He knew the _Cynesephon_. She was one of the "P" boats that in 1918 had been converted into a "Q" ship and altered to resemble a South American freighter. She was supposed to be the last word in mystery ships, but an opportunity to use her never arrived, owing to the Armistice. For certain reasons she had not been scrapped. She was now lying in one of the basins at Portsmouth Dockyard, snugly moored between two battleships of the _Thunderer_ class, which were permanently out of commission. And now the _Cynesephon_ was to be rescued from the scrap heap and reconditioned--why? Putting two and two together--the commissioning of the _Cynesephon_ and the Commander's inquiries about Cavendish's service on the South American station--the Sub made a shrewd guess.