The Sixth Doctor returns, and he's thrown straight in at the deep end! Travelling the galaxy with Melanie Bush and their brand-new companion, marine biologist Hebe Harrison, there are wonders to see, dangers to face and plenty of peril beneath the waves.

1.1 The Rotting Deep by Jacqueline Rayner (2 parts)
A mysterious SOS summons the Doctor and Mel to an oil rig in the North Sea where a dwindling group of survivors awaits rescue from a lethal menace. One of their number is Hebe Harrison, a wheelchair-using marine biologist who is definitely more than she seems. Can our heroes escape the rig? And just what is killing off the rig's beleaguered crew?

1.2 The Tides of the Moon by Joshua Pruett (2 parts)
For Hebe's first trip in the TARDIS, the Doctor and Mel take her to the nearest available 'water world' – the Moon, two billion years in the past! Its advanced humanoid inhabitants, the Gilleans, are terrorised each night by their monstrous enemies, the Sheega. Even more worrying is that in a matter of hours, this ocean world will be laid waste by the gravitational interference of the blue-green planet next door...

1.3 Maelstrom by Jonathan Morris (2 parts)
The Doctor, Mel and Hebe visit the archipelago world of Veludia, only to discover a planet beset by electromagnetic storms where three survivors traverse the seas in a ramshackle township. These survivors are 'corps' - bodies used as hosts for minds stored in the vessel’s ‘mind-drive’ - and the Doctor, Mel and Hebe are seen as welcome replacements. But there's something lurking in the waters below, something of rage and power that wishes to destroy them all: the Maelstrom!