A player cannot attack face-down monsters during this turn.
Rulings
Individual Card FAQs
If you activate "A Feint Plan" after your opponent declares an attack against one of your face-down monsters, that attack resolves as normal, but they cannot attack other face-down monsters for the remainder of the turn. (This is because "A Feint Plan" prevents declaring an attack against a face-down monster, and the attack has already been declared by the time "A Feint Plan" resolves during the Battle Step.)
Netrep Rulings
If all of a player's monsters are face-down in the turn that "A Feint Plan" is activated, the opponent may not attack directly, unless the attacking monster can inherently attack directly, such as "Inaba White Rabbit," "Jinzo #7," a monster equipped with "Shooting Star Bow – Ceal", or Toons.
Trap|
Normal