Both sides vulnerable.  
South deals.
                              NORTH
                              S: K J 5            
                              H: A           
                              D: Q J 8 4     
                              C: A Q J 4 3
            WEST                             EAST
            S: A                             S: 9 6 4 3      
            H: K Q 8                         H: J 10 9 7 4 
            D: A K 10 5 2                    D: 9 6 3             
            C: K 10 9 7                      C: 5           
                              SOUTH                         
                              D: Q 10 8 7 2
                              H: 6 5 3 2  
                              D: 7         
                              C: 8 6 2    
                  SOUTH    WEST    NORTH    EAST                  
                   Pass     1D      1NT     Pass
                   2S     Double    Pass     3D 
                   Pass    Pass     4S      Pass
                   Pass   Double  (All Pass)
           Opening Lead: KD
     An opening no-trump bid with a hand containing a singleton
is frowned on by theory and avoided in practice by all but a few
eccentrics.  But an overcall of one no trump is another matter,
as with the hand shown above.  North had no perfect action after
West opened one diamond.  His bid of one no trump was as least as
good as any of the alternatives.
     South's two spade bid was to play, but after East raised
diamonds North made a very aggressive leap to four spades and
West doubled on what appeared to be pure power.
     The opening lead was the king of diamonds.  The best defense
would be a club shift at trick two with the intention of giving
East a club ruff later in the play of the hand.  At this point it
might be a little difficult to find this line of play.  Instead,
West cashed the spade ace and continued with the eight of hearts
to the singleton ace in the dummy.
     The key to success for declarer is to play an honor from
dummy under the spade ace.  The five of spades is required to
provide an entry to the South hand.  But even if he makes that
play, what is the best way to proceed?
     He can draw trumps but then what?  If West discards
carefully, retaining one heart, two diamonds, and four clubs, the
contract will be defeated.
     The winning play is to lead the diamond queen from dummy at
the fourth trick, discarding a heart from the South hand.  West
does best to win and play a heart.  Dummy ruffs high.  The
bidding and play suggest that West's spade ace is a singleton, so
South finesses the spade eight, draws trump and finesses the club
jack.  When the jack holds he throws his last heart on the
diamond jack, ruffs a diamond and repeats the club finesse to
make his doubled game. 
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