The away team accidentally triggers an explosion which kills Lieutenant Marla Aster, leaving behind her orphan son. A mysterious being from the planet appears in the form of the late Marla Aster, claiming to be his mother. The being causes other illusions that are designed to make the boy feel more at ease with his strangely undead mother. When the crew catches on, they try reasoning with both the boy and the alien. Eventually they win out by pointing out that the apparition can never be his real mother, and that he has the other crew members to help him through his grief.
Grief is always a hard subject to tackle, but I think this episode does a good job of it. It helps that they use a kid as the focal point, so that grief can be simplified for the limited time we have to explore it, but Worf and teenage Wesley are also adeptly used. In fact, their interactions with Jeremy are a bit like what self-parenting might sound like. Like, a part of you feels one way, but another, more mature part of you knows better.
And then there's the underlying question of what makes something real. If believing were everything, then insanity wouldn't exist, but if hard facts were everything, then delusions wouldn't exist because there'd be no room for them. Jeremy is offered a tempting fantasy. What, exactly, makes that fantasy wrong? Is it that he would always know that it was just a fantasy, so he could never fully accept it and be happy? Is it that it only mimics a lost reality, and can't actually replicate it exactly? Man, I got philosophy in my Scifi TV!