What 1999 Sesame Street film sends Elmo into Grouchland?

The 1999 Sesame Street film that sends Elmo into Grouchland is The Adventures of Elmo in Grouchland.

The story begins when Elmo loses his beloved blue blanket after a disagreement with Zoe. While trying to retrieve it, he is transported through a swirling tunnel into Grouchland, a world of trash-loving Grouches ruled by the greedy Huxley.

Elmo learns that Huxley has taken the blanket to his castle on Mount Pickanose. With help from characters including Grizzy and Stuckweed, Elmo begins a quest to get it back. Meanwhile, Sesame Street friends try to find him, but Grouchland’s rules make asking for help illegal.

Directed by Gary Halvorson, the film was released by Sony Pictures on October 1, 1999. It was the second Sesame Street theatrical feature after Follow That Bird. The shorter title Elmo in Grouchland is commonly used, but the stored answer is the film’s full canonical title.