Mei Lee and her colleague Jason from Sealab, are exploring a recently-discovered underwater trench, the deepest ever found. Their sea-bubble comes under attack by a giant squid and loses contact with Sealab. Max is sent to rescue them, but he too is attacked and loses contact with Sealab. Ace and Jake are sent to NASA to use an experimental Venus Probe, as only the probe can withstand the pressure at that depth. Meanwhile Max awakens to find himself in the undersea city of Atlantis, ruled by Princess Diana. At NASA, Ace is reunited with his old Air Academy comrade Sean O'Connor, who still holds a grudge against him since they had an argument over a girl. Diana tells Max her civilization has survived for many centuries since the city sank, and that they want no contact with the surface world, so he, Mei Lee and Jason are to remain there forever. Seeking control over Atlantis himself, Diana's cousin Naro attempts to steal the Venus Probe, leaving Ace to defend the ship underwater, where