Every Unit was obscured by identical masks made of silver with a blank, unmoving expression of a humanoid face. They wore identical red robes which made differentiation between the sexes unclear. (PROSE: The Masks of Makassar)
Akimus Makassar used his mask as a processing centre, which meant he could control the mind of any Unit, as well as anyone else who wore the masks. The shared mind increased their mental capacity. Makassar invited representatives from other worlds to increase his command. In an attempt for Makassar to leave his body and take over the body of the Ninth Doctor as well, the Units placed the Doctor and Rose in the group mind.
The Doctor used his mind to free Rose from Makassar's hold and created a version of the Powell Estate out of his memories for them to hide in. When the Doctor realised a rogue Unit, Eln, had a connection to Rose when he tried asking for help, the Doctor came up with an idea. He asked Rose to ask him to get free again.
While the Units were distracted, the Doctor's projection flew through Makassar's castle and removed a mask from a Unit. Caught in a mental struggle with Makassar, the Doctor's projection succeeded in removing Makassar's mask and placed the other Unit's mask on his face. Makassar was caught in a loop and the Units and the representatives were freed. (PROSE: The Masks of Makassar)