It isn’t clear what a singleton jack is worth, but regardless, North should open the bidding. Most hands with ace-king-ace are worth an opening bid (maybe not with 4-3-3-3 shape).
An overcall can be made on the one level with a 5-card suit and as few as 8 or 9 HCP, but not with East’s hand. Two main problems: one, the suit is lousy and two, East is vulnerable. If East had, say, KQ10xx of diamonds and a side ace, he should overcall.
The response to 1 is 1 (South must show the 4-card major here).
West, shouldn’t even think of overcalling on the two level! His suit is okay, but his hand is flat and he is vulnerable. An overcall on the two-level would show a better hand—roughly opening-bid strength.
Since he has 4-card spade support, opener can now count distribution points. The singleton diamond is a huge asset, worth at least 2 extra points (some teachers might say 3). I wouldn’t also count a point for it being the jack. So, outside of diamonds, North has 12 HCP + 2 or 3 for the singleton makes about 14 or 15. This is just short of the requirements for a jump raise to 3, so opener has to be content to raise to 2 (showing up to about 15).
Responder doesn’t have enough to look for game, so he passes the raise to 2.
As much as I don’t like letting my opponents play in 2 of a major when they have found a fit, I can’t see East or West coming in. It is way too dangerous at this vulnerability and both of them have very flat hands.
These days, from an ace-king holding (against a suit contract), the recommended lead is the ace. That way, partner knows you also have the king (since laying down an ace without the king is a poor idea).
Declarer sees that he is very comfortable in 2, and it is only a matter of overtricks. It looks like he will lose only this one diamond trick (the other one can be trumped in dummy). If clubs behave, there will be only one trick to lose there. Likely a heart will be lost, and there is also the matter of the trump queen.
That Q matter might be resolved at trick 2. I can honestly say, that as West, I would shift to a trump. It looks like declarer will be ruffing diamonds in dummy and I want to prevent that. Unfortunately, this trump switch solves declarer’s mystery of the Q guess. C’est la vie. I hope West has an understanding partner.
Declarer should be in no hurry to trump his other diamond in dummy. After he wins the trump shift, he should play a second round of spades. There is an old saying that if both sides are leading the same suit (spades in this case), that somebody was wrong. Noting the 3-2 break, declarer should draw the last round. There is still no hurry to trump the diamond in dummy (you will see why later).
Next come the clubs. When they turn out to be 3-2, declarer knows he will be making at least 10 tricks (all he will lose is a diamond, a club, and maybe a heart).
What about those hearts? Declarer has a 25% chance available. If both the queen and jack are in front of the AK10, declarer succeeds by leading towards the AK10 for a deep finesse. If the QJ are with West, inserting the 10 will win the trick. Declarer might as well try it. If you carefully follow the play (and the timing), you will note that declarer can safely try for this extra overtrick. (Had he trumped the diamond first, upon winning the A, the defense would have made him use his last trump, and he wouldn’t be able to afford the loss of a heart trick). Alas, nothing good happens in hearts, so declarer loses one trick there and scores 170.
That should be an okay score. I wouldn’t expect other North-South pairs to reach game. Possibly some will make only 140 in spades (losing a trick to the Q). What I do fear is that some E-W pairs at other tables won’t be so disciplined. If either one of them at any point bids diamonds, they could get in trouble. If they compete to 4D, it is at least 200. If they play in 3 and it is doubled, that would also be 200.
1) A one-level overcall can be made with as few as 8 or 9 HCP, but in that case, it would need to be a good suit.
2) To overcall on the 2-level, especially vulnerable, a good suit and/or opening bid strength is required.
3) Try not to let your opponents play on the two level when they have a known fit; but, if you are flat and vulnerable, you have to be cautious about balancing.
4) With ace-king in a suit, lead the ace (against a suit contract).
5) When it appears declarer might be trumping losers in dummy, usually a trump shift is indicated.
6) Missing QJ in a suit, you have a chance for no losers if the AK10 are over the QJ.