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After being transferred to Dalby's civil intelligence unit from Major Ross's military unit, Harry Palmer is assigned to investigate the abduction of Dr. Radcliffe, a scientist, after the murder of his bodyguard. A number of scientists have been disappearing mysteriously, then reappearing with curious "brain drains." Radcliffe is especially wanted by the intelligence authorities because of a secret file in his possession, and Palmer investigates Bluejay, a crook who deals in the sort of merchandise represented by Radcliffe. When this trail begins to fade, another clue prompts Palmer to raid a deserted London factory; and he discovers a piece of recording tape on which the perforated word "Ipcress" appears. Meanwhile, Dalby makes a deal with Bluejay to exchange the brainwashed scientist for £25,000 in cash. Then one of Dalby's unit apparently penetrates the Ipcress mystery and is strangely murdered; and when yet another civil intelligence agent is killed under circumstances implicating Palmer, Dalby suggests that Palmer go into hiding. Before he can do so, however, he is abducted and brought to the factory to be brainwashed. He escapes and calls Dalby and Ross to the factory for a disclosure. There Dalby reveals himself to be a traitor and is killed.