The Lions nearly got off to the worst possible start when Argyle striker Leon Clarke was able to pick out David Norris on the far side of the box.
He centred to leave Vincent Pericard with an easy tap in but he missed the ball completely.
But the Lions failed to learn their lesson and they left the same man unmarked from a corner just nine minutes later.
This time Pericard made no mistake and he powered in a close-range header after Paul Connolly's pin-point ball in.
The New Den faithful were not about to stand for a third straight home defeat and they began to urge their team forward.
For the most-part, balls up to centre-forwards Ben May and Carl Asaba would not stick, but Plymouth's defence could do nothing when Tony Craig's pin-point ball found Alan Dunne on 22 minutes.
He tried to pick out the far corner of the net with a firm header, but visiting keeper Romain Larrieu got down well to his right to tip round the post.
The first real chance gave the Lions more confidence to create more and the equaliser was deserved when it finally came on the half-hour mark.
If one man was going to provide the spark Millwall needed it was impressive youngster Marvin Williams and he was alert enough to beat Larrieu to the ball and head home when an Asaba shot rebounded up into the air off a defender.
That was to be Asaba's only real contribution of the match and he trudged off the pitch to be replaced by Berry Powel on 41 minutes to jeers from the home fans.
Indeed, Powel could have had an instant impact if he had not mis-controlled when a Matt Lawrence ball fell at his feet in the box.
He did much better to control the next ball that came his way as he chested another Lawrence ball down to May whose rasping shot from the edge of the box was tipped over by Larrieu.
Two corners followed, the second of which Millwall thought they had scored from but Larrieu was being impeded when Zak Whitbread nodded in Dunne's ball and the goal was disallowed.
But despite some early second-half pressure, Millwall failed to capitalise with only a lone David Livermore shot troubling Larrieu.
The Lions now look condemned to becoming a League One club.