Portal 2 plot spoiled by casting ad

One of the key characters in what is presumably Portal 2 has been revealed in an advert for voice over actors.

Members of subscriber-only site Breakdown Express tipped Kotaku off about the vacancy, which is for "a new game in the Portal series from Valve Software". The game is said to feature "dark comedy", and recording will begin "tentatively end of July, 2008".

If you're worried about spoilers, stop reading now.

Cave Johnson, "eccentric dead billionaire", is the character in question. He is the boss of a successful "applied science" company (Aperture Science) who is used to getting what he wants, using his high-energy salesman-like personality to win people over.

But, alas, he "starts to lose his grip on humanity" as the story unfolds, and turns from "sidekick to principle antagonist" as he "isolates himself from the people around him".

Johnson was already sleeping in the ground in the original Portal game, suggesting that the sequel may be a trip back in time. Being your "sidekick" may also mean you are rather more than a lab rat this time around.

Valve has been contacted for comment, and has previously been quite clear about there not being another Portal game planned for this year.

Spokesperson Doug Lombardi has also promised that when a sequel does arrive, it will be "equally as revolutionary" as the first.

Portal was our favourite game in the whole of 2007, using its subtle charm to beat away competition from the likes of Super Mario Galaxy and BioShock.