Watson is trying to find out whether a telegram that Holmes previously sent to Barrymore from London was delivered to him personally. It is important to find this out as it will establish whether or not Barrymore was at Baskerville Hall at that time and not in London following Sir Henry, as Holmes suspects he might have been. These suspicions have only been strengthened by what Watson has seen of Barrymore personally at Baskerville Hall; he is a dark, brooding figure, and both he and his wife seem to be harbouring some guilty secret.
Watson's misgivings about Barrymore are not allayed at all by his visit to the postmaster. The postmaster can only tell him that his son delivered the telegram personally to Mrs Barrymore and not to her husband who, it seems, was up in the attic at that time. However, there seems no way of proving if he was in the attic, as claimed, and not somewhere else. As Watson observes: 'It seemed hopeless to pursue the inquiry any further' (chapter 7). Holmes's 'ruse' in sending the telegram has failed.