Ostensibly safely imprisoned on a remote island, the "Master" (Roger Delgado) proves that he can still get up to mischief as the visiting "Doctor" (Jon Pertwee) and his pal "Jo" (Katy Manning) soon find out. It's also around this time that the Royal Navy is having problems with sinking ships. They are bound to be connected, and so swiftly our intrepid time travellers have to find out just what their cunning foe is up to - and with whom! It doesn't take us long to discover that the eponymously ancient species, living in hibernation beneath the waves, have re-emerged bent on reclaiming their planet from us pesky apes. Maybe the "Doctor" can suggest some planet-sharing policy? Maybe the "Master" will be having none of that? That's the thrust of this six-parter that spends much of it's time out of the studio, at sea or in a diving bell, whilst providing plenty of opportunity for some superior spatting between the two Time Lords, some characterful opponents and the odd trace of an early evening moral dilemma. The UNIT folks have a series off for this one, so that leaves us supported by "Capt. Hart" (Edwin Richfield) and frustrated by the Whitehall mandarin "Walker" (Martin Boddey) whose pomposity and smugness manages to get up just about everyone's nose as he takes a typically sledgehammer to crack a peanut approach to a diplomatically delicate scenario. It really helps this that they've taken this out and about - it's less constrained than many and likewise, the writing allows the characters that bit more freedom too as duplicity and betrayal are never far from the gist of the action. One of Pertwee's better adventures, this one and not even an hint of Yvonne de Carlo.