I remember as a teen watching X-Men on Fox every Saturday as part of their Fox Kids line up. The cartoon was a true series for the first couple of seasons. Each episode was part of a larger continuing story that aired in order. Unfortunately, Fox messed that up when they began airing the episodes out of order -- a practice the network used resulting in the cancellation of Firefly.
In "Xavier Remembers" Professor X (Charles Xavier) is unconscious. He has a series of dreams that cause him to physically weaken. However, Hank and Jean believe someone or something is manipulating Xavier's mind. They are right, an old foe of Xavier's, The Shadow King, has come for payback. Years ago, Xavier defeated The Shadow King on the astral plane and exiled him there.
The Shadow King gains control over Xavier holding him hostage and using him as a shield. He knows the X-Men won't hurt kill him because he is in Xavier's body. Jean Grey sends her astral body into the astral plane to free Xavier. In the astral plane Jean and Xavier fight and defeat The Shadow King.
What Season and Which Episode is "Xavier Remembers"?
That is kind of hard. As I mentioned earlier in the post, Fox aired the episodes out of order beginning in season three. The placement of "Xavier Remembers" within the series differs.
From the Marvel Universe perspective, "Xavier Remembers" is in season three episode 23. Hulu sort of follows the Marvel Universe chronology placing the episode in season three, episode 24. Amazon and IMDB have the episode in season four episode 17/16 (respectively). If you go to look up the episode online, look it up by the episode title "Xavier Remembers" otherwise you might not find it.
Do you remember X-Men: The Animated Series?