K9 becomes confused about his memory loss when an alien "bounty hunter" arrives and teams up with an evil Department inspector called Drake. Ahab, the Bounty Hunter, claims there is a price on K9’s head for murdering a galactic peace delegate in the 50th century.

Panic ensues when a museum exhibit is stolen and K9, Starkey, Darius and Jorjie are on the trail of the culprit only to find a giant menace hiding in London’s abandoned sewers. However, is the Alien really a threat or is Drake the bigger evil?