Though an option plan doesn't protect you, once you've been granted a specific number of options (or rather, restricted stock), there is little anyone can do do take them away from you, short of firing you, without diluting the founders equally.
That depends on your class of equity! Your terms might be different to the founders - in the UK you can read the memorandum and articles to understand the current equity structure and dilution rules for a company.
(Changing the mem and arts usually requires a shareholder meeting, and can be contested in court if the interests of minority stock holders are not taken sufficiently into consideration.)
In addition even if you do hold stock of the same class as the founders that won't always prevent you getting screwed over. The board might sign up to a sucky deal through innocence, incompetence or collusion and condemn you as a side effect.
