Strange readings are detected by Spock while scanning the planet, Excalbia. These reading lead Spock to believe there is carbon cycle life forms on the planet. The scans are thought to be faulty because the surface is covered by a molten substance. However, the crew is surprised when deceased U.S. president Abraham Lincoln appears on the screen and invites them down.
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Warning: Star Trek season 3 episode 22 guide may contain spoilers