Penn, Boone, and Sashi must find the last mountain beast to protect their civilization from getting destroyed by Rippen and Larry before the fifth moon rises. However, Penn's MUHU tracks a shard in the dimension, distracting Penn from the mission.
It is revealed that Boone does not know how to say MUHU, in his place says "MUMU".
Penn and Sashi fight for the first time and hugged together.
The title refers to the 1984 Western novel The Last Mountain Man (and the subsequent series) by William W. Johnstone.
Middleburg • Middleburg Central High • The Odyssey • Fish Stick on a Stick • North Pole • Knight World • Superhero World • Arabian World • Plush Toy World • Clown World • Dinosaur Cowboy World • Cereal World • Old English World • Giant Monster World • Zombie World • Space World • Ocean World • Color World • Musical Fairy Tale World • Caveman Spy World • Future Barbarian World • Ultrahyperball World • Demon World • Alien World • Galaxy World • Temple World • Dragon World • Gothic Mystery World • Plant World • Animal Barbarian World • Toy World • Outer Space Court World • Robot World • Pirate World • Fairymania World • Steampunk World • Ball World • Ghost World • Rock, Paper, Scissor World • Beast World • Anime World • Sitcom World • Nothingness • Most Dangerous World Imaginable • Least Dangerous World Imaginable
Multi-Universe Hologram Uplink • Multi-Universe Transprojector • Pigoilet • The QPC Unit • Vortex Tops • Specs