The planet Murra was a military power which had intentions of invading the planet Earth. They learned to their dismay that the Legion of Super-Heroes had heard rumor of a pending invasion and discovered that the Legion intended to implement "Plan-R" to successfully defend against the invading forces.
Murra's military power sent three spies to infiltrate the Legion of Super-Heroes so as to steal the secrets of Plan-R. The spies took pills which gave them special powers, posed as Legion applicants, and were inducted as members. They found the plans and headed back to Murra.
What they did not know was that the Legion was on to them the whole time. The induction of the three spies was a ruse which allowed the Legion, and thus the United Planets, to locate and neutralize the planned invasion.