Killing Time (Star Trek, No. 24)

Second History: a Romulan time-tampering project that has transported the Enterprise and the galaxy into an alternate dimension of reality. Now Kirk is an embittered young ensign and Spock is a beseiged Starship commander. Lured into a Romulan trap Captain Spock and Ensign Kirk must free themselves from both their captors and their own altered selves...before the galaxy hurtles toward total destruction!