The death of Commander Trip Tucker in 2161, when he sacrificed himself to stop intruders threatening Captain Archer shortly before the signing of the Federation Charter.
The death of Trip Tucker is shown in These Are the Voyages, the final episode of Enterprise. The events are seen as a holodeck recreation run by William Riker aboard the Enterprise-D in 2370.
In 2161 Enterprise NX-01 is returning to Earth for the signing of the charter when Shran asks for help rescuing his young daughter Talla from former criminal associates. The rescue succeeds, but the kidnappers later board the ship and threaten Archer.
Tucker lures them to a plasma junction and overloads it. The blast kills the intruders and leaves him fatally injured, and he dies in sickbay. Archer goes on to give his speech at the ceremony, with T'Pol and the others mourning their friend of ten years.