Perhaps the essential three-parter in the fourth season, the Vulcans take center stage in Enterprise for the first time since the first season.
"The Forge" kicks things off in dramatic fashion with a terrorist attack (other than Deep Space Nine, Enterprise is the place to look in Star Trek if you want to look at the franchise's thoughts on this prescient topic, and its conclusions may surprise you) that claims the life of recurring character Admiral Forrest.
The attack draws attention to a group of radical Vulcans with ties to the historical figure of Surak, who came up with all the central tenets of logic that have been overlooked or distorted since his time. Critics of the show's handling of Vulcans saw this as an opportunity to make amends, though I don't think you need to adhere to that interpretation to enjoy this historic moment.
T'Pol is necessarily at the heart of this story, complicated family connections and such.
As with any multi-part episode, "The Forge" is not much of a complete experience, so you need to keep watching.
franchise * series * essential * character
Memory Alpha summary.