The "Master," portrayed by Roger Delgado, demonstrates he’s still capable of causing trouble despite being seemingly contained on a secluded island, as the "Doctor" (Jon Pertwee) and his companion "Jo" (Katy Manning) quickly discover. Simultaneously, the Royal Navy is experiencing issues with ships mysteriously sinking. These events are inevitably linked, prompting the time-traveling duo to investigate the "Master’s" schemes and identify his allies. It's not long before it becomes clear that an extremely old species, previously dormant beneath the ocean, has resurfaced with intentions of taking back their world from humankind. Perhaps the "Doctor" could propose a plan for sharing planets? Or maybe the "Master" would reject such an idea entirely? This narrative arc drives this six-part story, which spends considerable time on location—at sea and within diving bells—while also providing ample opportunity for sharp verbal clashes between the two Time Lords, memorable adversaries, and touches of a familiar early evening moral quandary. UNIT’s involvement is limited in this installment, introducing "Capt. Hart" (Edwin Richfield) as support and "Walker" (Martin Boddey), a Whitehall official whose arrogance and self-satisfaction irritates nearly everyone while he applies an overly forceful approach to a sensitive diplomatic situation. The decision to film on location significantly enhances the story; it feels less restricted than many others, allowing for greater character development because deception and disloyalty are constant elements of the plot. This is among Pertwee’s stronger adventures, thankfully without Yvonne de Carlo's involvement.

Source on TMDB