User goes first with the opponent's attack at 1.5x power.
The user copies the move that the target has selected in that turn and uses that move itself, with the power boosted by 50%. Me First will fail if the target moves before the user, or if the target uses a non-damaging attack. If Me First is used on the recharge turn of a move that requires the user to forfeit a turn after its use, then Me First will copy said attack regardless. This move cannot be blocked by Substitute.
So, it is 50% of the Base Power of the move.