Ocean Pioneer, a new automated chemical tanker, sails into a bank of mysterious fog and explodes, apparently without cause. A year later, Ocean Pioneer II, christened by Lady Penelope Creighton-Ward herself, casts off from Southampton. Investigating the original explosion, Brains discovers that a freak reaction between the tanker's cargo of liquid alsterene and the sea fungi OD-60 is responsible for Ocean Pioneer I's plight. Thunderbirds 1 and 2 speed to the rescue as Ocean Pioneer 2 enters another fog bank....