Star Trek - The Original Series - The Conscience of the King
Capt. James Tiberius Kirk is contacted by an old friend from Tarsus IV colony who tells him that the man who carried out the massacre when they were there is back. The man behind the massacre Kodos is now masquerading as Karidian, a circus performer. Kirk doesn't believe him until Leighton is killed. As a result, Kirk invites the performers to appear on the USS Enterprise NCC-1701A. Karidan agrees.