In case you skip it...

In this episode, the Enterprise is sent to investigate and retrieve the Hood, a sister ship whose life support systems suddenly and catastrophically failed. Before Scotty can determine why that happened, the Hood’s engines overload, destroying the ship and nearly killing the investigating away team.

Chekov manages to save the team through an act of quick-thinking bravery and ingenuity, so Scotty promotes him from ensign to lieutenant junior grade, denoted by giving him new insignia braids for his uniform.

The cause of the Hood’s destruction and the death of its crew is left a mystery.