The Halloween Tree is a 1993 feature-length made-for-TV cartoon movie based on the 1972 fantasy novel of the same name by Ray Bradbury.
The story centers on a group of children all dressed up and excited for a night of trick-or-treating. When one of their friends, Pip, goes missing, the gang sets out to look for him, thinking the boy is just playing a joke on them. On their quest to find Pip, they end up going through a forest and into a mansion where they met Mr. Moundshroud (Star Trek‘s Leonard Nimoy). Moundshroud is concerned after learning that the children don’t know the history behind the costumes they’ve picked to wear for Halloween, but eventually the kids go back in time to various eras and countries to learn the origins of Halloween and also save Pip.
The entire cartoon is up on YouTube in seven parts, which I’ve embedded here below. While The Halloween Tree was released on VHS in the 1990s, there’s no DVD version, nor does there seem to be any plans for one at this time.
By the way, in the Part 2 video, Mr. Moundshroud says that Pip has “escaped to the Undiscovered Country” (The Undiscovered Country is the title of the sixth Star Trek, which starred Nimoy).