Playing imps, the auction went (West dealer, EW vulnerable):
1NT - 2♣* - pass - 2♦*
pass - pass - Dbl - pass
2♠ - pass - pass - pass
2♣ shows either ♦ or both Majors
2♦ is "Pass or Correct"
Partner leads ♦J and you see following dummy:
Obviously you take ♦A at trick 1.
How do you plan the defense?
Question is how you'll defeat this contract. EW have 22-24HCP, this leaves partner with 7-9HCP. You already know partner doesn't have ♦K or ♦Q. LHO might have passed with 4 ♦s from KQ, so he probably has only 3 ♦s. This also explains partner's intervention: a poor but long suit. So his values must be somewhere else. The only way to defeat this contract seems if he has good trumps and an Ace. Partner will have 2 trumps at most, so chances are low. However, ♥A or ♣A or ♣K won't run away on declarer's ♦s. We can overruff dummy, so I think the best opportunity we have is to continue a ♦ and hope.
Here's the full deal:
This is what should happen: declarer plays a ♥ to the K and plays a low trump. You play low and declarer inserts the Ten for partner's Jack. Partner also knows you don't have any ♦s left so he'll play his lowest remaining ♦ to show a ♣ suit preference. You can ruff with the 8, return a ♣ to partner's Ace who returns another ♦ (to promote his ♠K if you have the Q) which you ruff with the Ace. Even better!
Only a dummy reversal for the defenders gets this contract down. ♦A, ♣
A and 4 ♠ tricks is just enough.