The Doomsday Machine

Following a trail of shattered solar systems, the Enterprise finds the USS Constellation crippled and adrift, with its commander, Commodore Matt Decker, alone aboard and in shock. He explains that a gigantic machine which devours planets for fuel attacked his ship. He beamed his crew down to a planet for safety, only to watch the machine destroy it. Spock concludes that it is an ancient doomsday weapon from outside the galaxy. Decker is sent to the Enterprise while Kirk stays on the Constellation with Scotty. Driven by guilt and revenge, Decker uses his rank to take command and attacks the machine, almost losing the ship. Spock relieves him, but Decker steals a shuttlecraft and flies it into the machine's mouth, dying in the attempt. The blast does some damage, and Kirk realises a bigger explosion could finish the job. He sends the Constellation in, set to self-destruct, and is beamed out just before it destroys the planet killer.