The Tides of the Moon

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...