One day Harold, while sailing in the channel, was driven by tempest on the territory of Guy, Count of Ponthieu, who, being informed of his quality, immediately detained him prisoner, and demanded an exorbitant sum for his ransom. Harold found means to convey intelligence of his situation to the Duke of Normandy; and represented that he had met with extremely harsh treatment from the mercenary disposition of the Count of Ponthieu, who was William's vassal.
William was immediately sensible of the importance of the incident: he foresaw that if he could once gain Harold, either by favours or menaces, his way to the throne of England would be open, and Edward would meet with no further obstacle in executing the favourable intentions which he had entertained on his behalf. He sent, therefore, a messenger to Guy, in order to demand the liberty of his prisoner; and that nobleman, not daring to refuse so great a prince, put Harold into the hands of the Norman, who conducted him to Rouen. William received him with every demonstration of respect and friendship; and even persuaded him to take part in a campaign which he was waging with Count Conan of Brittany, in which Harold highly distinguished himself, and was rewarded with the compliment of knighthood. In his anxiety to be allowed to return home, Harold appears to have compromised himself by taking some sort of oath, which William attempted to make additionally binding by a curious trick. He caused Harold, when he swore, to place his hand on a chest, and then, withdrawing the cover, showed the Englishman the relics of the saints, which had been collected from all parts of Normandy. This device is quite in keeping with the ideas of that time, and any breach of the engagement would have been considered a most wicked act of perjury, even though the taker of the oath was not aware of the solemnity of the promises he had made.
The terms of the oath are quite uncertain. Harold remained, so far as we know, absolutely silent on the subject, and his silence naturally conduces to the belief that he must have made some stipulations that he had no right to make. Professor Freeman strives hard to prove that he only promised to marry William's daughter, and that he did homage to him as his future father-in-law. The Norman chroniclers assert that he did homage to William as his future king, and promised in the meantime to deliver to him the castle of Dover, and to marry his daughter. It is hardly credible that Harold, though his position was a very difficult one, can have compromised himself in this manner; but it is possible that he may have thought that no price was too heavy to pay for freedom, and may have consoled himself by reflecting that to pledge himself to William as his future king was perfectly illegal, inasmuch as the election lay in the hands of the Witena-gemot. In any case, whatever understanding was concluded, Harold made no attempt on his return to carry it out, but continued in that line of conduct by which he accustomed the people of England to regard him as their future sovereign; and broke one of the conditions, at any rate, by marrying the daughter of Ælfgar, the widow of Griffith of Wales. He was so completely successful that the Confessor, on his death-bed (he died on January 5th, 1066) requested the Witena-gemot to choose Harold as his successor, and said nothing concerning William of Normandy, or any promises that had been made to him at the time of his visit to England.