This deal is from an online EBU duplicate.
North led the spade jack against three notrump, ducked to West's king.
Declarer now ran the club ten. Without much clue as to what declarer's shape or honour location was South ducked the club smoothly.
Declarer now played the heart king and then the heart jack which both held. If declarer had known the layout he could have cashed out for an overtrick now.
But he assumed the club finesse was working and played the nine of clubs to the jack and queen. South now played a diamond through; West had no more clubs or hearts to get to dummy with, so tried the eight of diamonds. North won and played another spade through for South to take four spade tricks to defeat the contract by two tricks.