S1:E1-2 Broken Bow: Captain Archer must assemble his crew to return an injured Klingon to Qo'noS
Let's start at the very beginning because it's a very good place to start! Humanity is introduced to its first Klingon in the form of Klaang, who crash lands his scout ship in the corn fields of a farm in Broken Bow, Oklahoma (hence the episode title) when his ship is forced down by a couple of Suliban pursuers.
The Vulcan High Command believes killing Klaang and sending his body back to Qo'noS would be the logical (and honorable) option, however, the inexperienced humans believe that to be barbaric and ultimately decide he should be returned alive so that we can make peaceful contact with the warrior race. Captain Archer needs to scramble to assemble his crew for his new ship's maiden voyage ahead of schedule.
Along the way, Klaang gets kidnapped and Archer needs to get him back to complete his mission, which takes us to Rigel X, where the Enterprise crew get their first real interactions with a number of different alien species. So instead of slowly getting their feet wet in meeting new alien species, we are thrown into the deep end and make our introduction to interstellar space at rapid fire. A great two-parter that covers a lot of ground in terms of establishing characters and gives the Enterprise crew a taste of what's to come.