I Am Klingon

Summary
The Enterprise-D discovers a century-old Klingon "lifeboat," with an unusual looking Klingon aboard.
 * Internal references place this story in 2367.
 * This story offers an answer to the "Klingon Forehead Question," which has since been contradicted canonically by ENT episodes "Affliction" and "Divergence".

Canon

 * "Errand of Mercy" (TOS episode)
 * "The Trouble with Tribbles" (TOS episode)
 * "Day of the Dove" (TOS episode)
 * "The Emissary" (TNG episode)
 * "Trials and Tribble-ations" (DS9 episode)