Rick's old drinking buddy has been dethroned and imprisoned by the new queen that Rick told him not to marry. Rick takes Morty on a mission to restore his buddy's throne, supposedly because Morty's such a great travel companion. Of course, Rick doesn't tell Morty that the aliens on this planet are very germ-phobic and he's inoculated Morty with dozens of infectious diseases to sneak by them. It's not like Morty's going to DIE or anything... probably.
Tom King Assures, He's Still Writing His 100-Issue Batman Story