David Marin is a powerful government appointee. When Wade Trask, a controversial and ambitious scientist, is condemned to die for treasonous remarks, Marin makes the mistake of going to visit him. Trask blasts him with a ray gun (that's right, a ray gun) which conveys the mind of Trask into Marin's body, and the mind of Marin into Trask's body.
Marin awakes to find himself inside Trask's body. Everyone who sees him will recognize him as Wade Trask, a seditious criminal condemned to die in five days.
All citizens are marked on the muscle of one shoulder with some sort of tracking device/mark which allows the government to target them, causing them ever increasing pain. In this way, the government can becripple any condemned citizen, then seize him. For this reason, condemned miscreants are not imprisoned-- they can be immobilized via this method. So-- even though Marin now looks like Marin (because of the mask) his body is still the body of Wade Trask, and the government has got him in a bind, scheduled for execution.
People are very suspicious of Wade Trask, so Marin cannot travel the country looking like Trask. So he finds a stopgap measure, a mask which matches his features. This allows Marin to travel around and work to try to safe his own life (looking like David Marin).
This novel has an exciting story, but some really bad writing. The prose is inornate and easy to read, but the plot elements are often goofy and downright imbecilic. For example, in a subplot, the main group (including Marin) has subdued the neighboring Jorgians in a military conflict. The great leader is due to accept the surrender of the Jorgian queen. Then she falls into his arms and he must do his duty by "satisfying her" ! What? How idiotic can it get?
Better science fiction is abundant, and there are better books by Van Vogt.
Commonly admitted as superior novels are the Van Vogt books Slan, Weapon Shops of Isher, Weapon Makers, and The World of Null-A.