Caretaker boss Gareth Ainsworth was looking for his first win in charge of the Chairboys.
Matt McClure gave Wycombe the lead after 11 minutes, heading home an Ainsworth cross as the hosts looked for their second win of the season in League Two.
But Madjo's header from the excellent Alex MacDonald's cross levelled it up after the break and earned Carl Fletcher's men a point, extending their unbeaten run to four matches.
Ainsworth, 39, cut inside Robbie Williams for the opener and delivered a measured cross that McClure headed beyond Pilgrims keeper Jake Cole.
Jo Kuffour's goal-bound shot was blocked before Stuart Lewis shot wide for the home side, while at the other end MacDonald's cross was headed wide by Onismor Bhasera with Jordan Archer beaten.
The visitors levelled the match in the 50th minute, Madjo guiding his header past Archer from MacDonald's right-wing cross.