The Leprechaun (Warwick Davis) finds his bride-to-be, Princess Zarina (Rebekah Carlton), in Dr. Mittenhand's (Guy Siner) laboratory, where he was about to use some of her blood to make himself a whole being again, due to its regenerative capabilities. The Leprechaun adds a tarantula and a black scorpion into the blood and injects it into him, eventually transforming him into a mutant arachnid that he dubs Mitten-spider, and his mind begins to regress into a more animal-like state. The Leprechaun later learns from Zarina that his gold lay in the cargo bay, so they head there to get it, but along the way he activates the ship's self-destruction sequence so no one would follow him to the gold. While in the cargo bay, he and Zarina have a falling out, resulting in him knocking her out.
The sole surviving marines- Malloy (Brent Jasmer), Tina (Jessica Collins), and Sticks (Miguel A. Nunez Jr.) -follow the Leprechaun to the cargo bay, but when they notice that the self-destruction sequence has been activated, Sticks is sent by Malloy and Tina to Mittenhand's laboratory to deactivate it on his computer while they went to kill the Leprechaun. Shortly after Sticks arrives in the lab, Mitten-spider ties him up in some webbing, but he manages to access the computer anyway, and learns that the sequence can't be de-activated without a password. He calls up Malloy and Tina- who are in a shootout with the Leprechaun ?and tells them this, so Tina decides to aid Sticks while Malloy fought the Leprechaun. However, when she tries to get the Leprechaun with a surprise attack and fails, she accidentally turns on a shrinking-enlarging machine, and a ray from it hits the Leprechaun and makes him grow into a giant. Malloy sends Tina on her way, and he tries to figure out a way to finish the Leprechaun off.
While heading to Sticks' location, Tina escapes an attack by Mitten-spider, but when she arrives in the laboratory, Mitten-spider attacks her again. She soon kills him by freezing him with liquid nitrogen and then blowing him to pieces with gunfire. Meanwhile, Malloy manages to evade the giant Leprechaun and get Zarina out of harm's way, but before he does he has Tina open the outer doors of the cargo bay, causing the Leprechaun to be sucked out into space where his body expands, retracts, and then explodes, much to Zarina's delight. Malloy then goes to Tina and Sticks, and together they correctly guess the password- Wizard, one of Mittenhand's descriptions of himself -to shut down the self-destruction sequence on the ship in the nick of time.
As they all hug in victory (with Malloy and Tina kissing), they look at the Leprechaun's remains floating through space, and his severed hand gives them the finger.
Thanks Tornado Dragon!