All right, the last time I played this was in a forum at my alliance's page in an online wargame. The objective is to post your preferred group--pirate, ninja, alien, or zombie--and with an action that defeats the prior poster's group.
Post 1: Ninjas rule!
Post 2: My aliens beam your ninjas aboard and perform unspeakably horrific experiments on them.
Post 3: Zombies aren't affected by alien mind control--they have no minds--and run rampant on your ship, then survive the crash, because they are already dead.
Post 4: Pirates have sharp thingies with which to slash your slow-witted zombies to pieces.
Post 5: The ninjas blow blinding powder in the pirates faces, then make short work of them.
You can reply to any group with a different group, but not the same group twice in a row.
I shall begin with...
Ninjas. They'd make a mighty show of their swordsmanship, but they take their nigh-invisibility seriously.