ENT: Impulse

The Enterprise receives a distress call from a Vulcan ship formerly lost in the Expanse. When the landing party arrives, they discover they have walked into a Romero like setting of crazy, pustulant Vulcans. As their Night of the living Vulcans continues, Phlox discovers the Vulcan madness is caused by Trellium D, the only substance which can shield the ship from the Expanse's anomalies. The episode ends with Archer assuring T'Pol they will find a way to shield the ship AND keep her sane.

ENT: Rajiin

In Our Mrs Archer, the search for both the elusive Xindi and and anomaly shielding trellium D brings the Enterprise to a trading post, where Trip gets a formula for trellium D and Archer gains a mysterious slave woman. As Trip and T'Pol heat up the shielding formula ( among other things) we learn that Saff....Rajiin has more to her than meets the eye.

TOS: Return of the Archons, Space Seed

In Return of the Archons, the Enterprise is sent to investigate the disappearance of a Starship crew. Aside from being another one of Roddenberry's thinly veiled attacks on religious thought (your god is a computer, see!), it recycles the classic "Kirk blinds the computer with illogic' plot twist.

Space Seed is THE classic TOS episode. Even my non Trekkie grandma would watch that one ( no reason to ask why). A wonderful episode with a wonderful actor, which spawned ( in this re viewer's opinion) the best of the Star Trek movies.