A plug tool is presented to selectively plug a back pressure valve (BPV) in a tubing hanger associated with a wellhead of a well. The plug tool and the BPV can seal bottom hole pressure from the well during removal of the Christmas tree, and can retain control of the well while the Christmas tree is removed. The plug tool and the BPV allow a blowout preventer (BOP) to nipple-up and be tested without removing the BPV and without replacing the BPV with a two-way check valve. The plug tool has lower threads to engage internal threads of the BPV, and upper threads to be engaged by a retrieval tool. The plug tool has a seal to seal with the BPV above the internal threads and below a notch.