Clevinger goes missing on a milk run to Parma. Everyone assumes he is dead. Yossarian thinks he has gone AWOL (absent without official leave, so no permission to go).
Wintergreen has a history of going AWOL. Every time he does, he is caught and brought back and made to dig holes as punishment. He doesn't mind the punishment so he keeps on going AWOL, but he confides in Yossarian that at some point Catch-22 will catch up to him and he'll go to jail.
One day while Wintergreen is digging his holes, he hits a water pipe and almost drowns to death. Someone mistakes it for oil and soon the whole base is digging.
Chief White Halfoat is kicked off the base. But he eventually comes back due to some loophole in the paperwork.
Appleby tries to set up a meeting with Major Major to accuse Yossarian of not taking his Atabrine tablets, but the sergeant tells him he can't see Major Major. Appleby leaves embarrassed.
Change of topic. Sergeant Towser gets uneasy whenever anyone mentions the dead man in Yossarian's tent. He knew the man; the guy's name was Mudd, and he died in combat before ever reporting for duty. (Because Mudd never officially checked in, he can never leave.)
A week after Mudd was killed, the men get news that they have a mission in Bologna. Because this is so dangerous, all the men try to get out of it by calling in sick.
The administration responds by shutting down the hospital ward so nobody can call in sick.
Yossarian is mad with fear. Well, madder and more fearful than usual. He thinks he's going to die. OK, so that is like usual.