An alien comprising of pure energy and which feeds on hatred manipulates events to cause a war between the crew of the Enterprise and some Klingons.
Star Trek TOS episodes centred around encounters with Klingons (or Romulans) are always worth a watch, this one because of the high level of action, and its memorable Klingon enemies, Kang (Michael Ansara) and his wife Mara (Susan Howard). From the get go, Kang and Kirk are at each others throats, the hostility between races heightened by the mysterious alien being, which is seen as a shimmering ball of light.
After facing off against each other on an uninhabited planet, the fighting continues on board the Enterprise, the alien ensuring that both sides are perfectly matched so that the fighting will continue indefinitely. The injured are miraculously healed so that they can quickly return to the fray.
Eventually, Spock and Kirk identify the alien and realise that they are being manipulated to provide it with the hatred on which it feeds. In order to starve the creature, they must bring about a truce with the Klingons, which isn't going to be easy...
It's comforting to think that bigotry, hatred and war could all be the result of interference by an extraterrestrial being, thereby excusing the human race for its violent behaviour. Sadly, I think we only have ourselves to blame and it's unlikely that slapping each other on the back and guffawing loudly will bring an end to intolerance and war any time soon. Still, it's nice to see Klingons and humans getting along for a change.