NL50 KK faces overbet on flop

[PokerFun007]

PokerStars No-Limit Hold'em, $0.50 BB (8 handed) Hand History Converter Tool from FlopTurnRiver.com (Format: 2+2 Forums)

UTG ($51.50)
Hero ($72.05)
MP1 ($29.50)
MP2 ($122.85)
CO ($21.80)
Button ($57.05)
SB ($14.95)
BB ($79.20)

Preflop: Hero is UTG+1 with K[img]/images/graemlins/heart.gif[/img], K[img]/images/graemlins/diamond.gif[/img].
<font color="#666666">1 fold</font>, <font color="#CC3333">Hero raises to $2</font>, <font color="#666666">4 folds</font>, SB calls $1.75, BB calls $1.50.

Flop: ($6) 4[img]/images/graemlins/heart.gif[/img], 2[img]/images/graemlins/heart.gif[/img], J[img]/images/graemlins/diamond.gif[/img] <font color="#0000FF">(3 players)</font>
SB checks, BB checks, <font color="#CC3333">Hero bets $3.5</font>, SB folds, <font color="#CC3333">BB raises to $20</font>,

Hero?

I think this is a set a great deal of the time, or at least a monster draw like AJh. What would you do?