Dragon Ball Z: Lord Slug^{[lower-alpha 1]} is a 1991 Japanese animated science fiction martial arts film and the fourth Dragon Ball Z feature film. It was originally released in Japan on March 9 between episodes 81 and 82 at the Toei Anime Fair as part of a double feature with the first Magical Tarurūto-kun film. It was preceded by Dragon Ball Z: The Tree of Might and followed by Dragon Ball Z: Cooler's Revenge.

Gohan visits Piccolo and shows him a new tune he has learned to whistle. Due to his advanced Namekian hearing, this causes Piccolo pain so he angrily orders Gohan to stop when they both sense an approaching threat incoming from outer space. Bulma and her father discover that a meteor harboring lifeforms is heading toward the Earth and will destroy the planet upon impact. Amidst mass panic, Goku and Krillin rush to intercept the meteor and hope to push it away by blasting it with Kamehameha waves but they are both rendered unconscious by the force of the meteor but it seemingly explodes in orbit. A spaceship then lands in one of Earth's cities.

An army of humanoid alien soldiers exit the ship and declare the Earth under the rule of their leader, Lord Slug. The soldiers attack a group of civilians but Gohan arrives to defend them. From the ship, Lord Slug observes that Gohan has a magical Dragon Ball sewn onto his hat which he loses during the scuffle. After his henchmen render Gohan unconscious and reveal the plan to convert the planet into a biological spaceship, Slug appears and uses telepathy to read Bulma's mind and learns about her radar used for tracking the Dragon Balls. Slug's men successfully gather the Dragon Balls and summon the eternal dragon Shenron who grants Slug his wish for eternal youth. Slug's forces begin transforming the Earth which begins to cause life to perish across the planet. Goku and Krillin are revived by Yajirobe who gives them each a healing Senzu bean. Gohan meanwhile resumes his assault against Slug's army and is rescued by Piccolo. Piccolo kills the henchman Wings and Gohan is overwhelmed by Medamatcha leading to Piccolo being injured when he jumps in front of an energy blast intended to kill Gohan. Goku and Krillin arrive and Goku easily kills Medamatcha and Angila. Slug appears and is attacked by Krillin who is quickly dispatched. Goku fights Slug but is overwhelmed; in a moment of rage, Goku is seemingly able to access a portion of the Super Saiyan form.

Goku pummels Slug and breaks his arm. Before Goku's eyes, Slug rips his arm off and regrows it, and removes his helmet, revealing that he is actually a Namekian. Through telepathy, King Kai warns Goku that Slug is a Super Namekian, a bloodline of violent warriors obsessed with power who were banished from their home world. Slug assumes a giant form and begins to crush Goku between his massive hands. Piccolo intervenes to rescue Goku and before he is also crushed, tears off his own ears and calls out for Gohan to start whistling. The whistling deafens and weakens Slug, allowing Piccolo to transfer his remaining energy to Goku who powers up and manages to fly straight through Slug's abdomen and incapacitates him. Goku then ascends into the sky and begins preparing a Spirit Bomb in order to destroy the pods transforming the planet, but Slug manages to pursue him. Goku launches the Spirit Bomb at Slug which sends him hurtling into his own machines, killing him and saving the planet. Yajirobe heals everyone with Senzu beans as they celebrate their victory.

• OP (Opening Theme):
  • "Cha-La Head-Cha-La"
    • Lyrics by Yukinojō Mori
    • Music by Chiho Kiyooka ^[ja]
    • Arranged by Kenji Yamamoto
    • Performed by Hironobu Kageyama
• IN (Insert Song):
  • Kuchibue no Kimochi (口笛の気持ち, "The Feeling of Whistling")
    • Lyrics by Dai Satō
    • Music by Chiho Kiyooka ^[ja]
    • Arranged by Kenji Yamamoto
    • Performed by Hajime Ueshiba
• ED (Ending Theme):
  • "Ya" na Koto ni wa Genki-Dama!! (「ヤ」なことには　元気玉!!, "There's a Genki-Dama in Bad Things!!")
    • Lyrics by Dai Satō
    • Music by Takeshi Ike ^[ja]
    • Arranged by Kenji Yamamoto
    • Performed by Hironobu Kageyama with Shines

At the Japanese box office, the film sold 3.6 million tickets^[3] and grossed ¥2.21 billion^[4] ($16.4 million).^[5]

It was released on VHS in North America on August 7, 2001, and on DVD on September 4, 2001. It was later released in Double Feature set along with The Tree of Might (1990) for Blu-ray and DVD on September 16, 2008, both feature full 1080p format in HD remastered 16:9 aspect ratio and an enhanced 5.1 surround mix. The film was re-released to DVD in remastered thinpak collection on November 1, 2011, containing the first 5 Dragon Ball Z films.^[6]