A cryptic message on the Teen Titans' bulletin board heralds the arrival of Speedy to their secret lair. The adventure seeking bowman is just in time to accompany the Teen Titans on their next assignment. Responding to a distress letter, sent by teen genius, Willie Gregson, the Teen Titans travel to the island laboratory of Doctor Simon Finley. Upon arrival, the team switches to their civilian identities, so as not to attract undue attention. On the lake, they meet with an old fisherman, who tells them the tale of the Lamprey Monster. Aqualad investigates further, and discovers that the fisherman's reel is actually a phone line. Sending a signal, the fisherman summons the Lamprey Monster. Aqualad grabs the team's raft, and swims off at super-speed. The Lamprey Monster belches out fiery projectiles. Despite Aqualad's best evasive maneuvers, the raft is hit, sending the team into the lake. Reaching shore, Robin inspects the damage to the raft.
The Teen Titans overhear the fisherman threatening Gregson. Following Gregson back to Finley's laboratory, the Teen Titans surreptitiously observe Gregson steal a file from Finley's safe. The Teen Titans confront Gregson, who explains that he's being blackmailed into delivering the file to the fisherman. Removing one page from the file, Robin tells Gregson to deliver the file. Gregson places the file inside a mooring float. As the Teen Titans watch, the float suddenly rushes out into the lake. Speedy fires an arrow, attached to a line, into the float, immediately getting dragged, along with it, into the lake. Aqualad catches up to Speedy. Soon the pair encounter the fisherman, reeling in the float. Speedy launches a volley of arrows into the fisherman's boat, sinking it. As The Teen Titans close in to recover the fisherman, and the float, the Lamprey Monster surfaces again.
Wonder Girl ties her lasso around the Teen Titans' speed boat, and tows it out of harm's way. Riding atop Aqualad, Speedy snags the float with a towline, attached to an arrow. Speedy fires the float back to shore. As the Lamprey Monster closes on the Teen Titans, Speedy hits it with an explosive arrow, that destroys it. The Lamprey Monster is revealed to be merely a ship, helmed by crooks. In scuba gear, the criminals make for the island, where they catch up with Gregson, Finley, and the file. Gregson takes a spear gun shot meant for Finley. KId Flash subdues most of the gang. With the file in his grasp, the gang's leader makes it to his escape boat, only to have Speedy sink it with a "torpedo arrow". Wonder Girl recovers the gang leader, and the file, from the surface of the lake. With the file out of the wrong hands, Finley forgives Gregson for his part in the criminal enterprise.