South opened One Heart and North contented himself with a raise to Two. South made a try for the heart game by bidding Three Diamonds and, although he had an unfavourable holding in diamonds, North went on to game on the strength of his maximum single raise on the first round.
West led the eight of spades against Four Hearts and the king lost to the ace. The jack of diamonds came back but the finesse lost and West led another diamond. South could take one discard on the queen of spades but there was no way of avoiding the loss of another diamond and a club.
So what was it that South had missed? The spade position looked clear after the lead of the eight of spades and it was his play to the first trick that proved costly. He should have played low from dummy instead of putting up the king.
East wins with the jack of spades and returns a diamond. Declarer can afford to finesse because, later on in the play, he can take a ruffing finesse in spades which will be worth two tricks to him; thus both losing diamonds can be discarded.