This season has 13 regular episodes.
New characters introduced include Captain Deadwood, a sailor made entirely of wood, except for the little finger; Fred Fredburger, an incorrigible dumb, elephant-like creature from the Underworld, who only cares about nachos, frozen yoghurt, spelling his own name, and saying "Yes!"; and Pandora (a parody of Dora from Dora the Explorer), who tricks Mandy into opening Pandora's box.
Other characters that have major appearances are Judge Roy Spleen, the judge of the Underworld Court; Miss Butterbean, who is kidnapped by the unfrozen Fred Flintstone (from The Flintstones); General Skarr, who becomes a plant-like creature while tending to his garden; Hoss Delgado and Eris who became a couple, and now break up; Morg, the Martian Grim Reaper, who, together with Grim, plans to conquer Earth; Dracula, who is as cranky as always; Principal Goodvibes, who becomes an gangsta-rapper; Irwin's grandmother, who knows plenty of yo-mamma jokes; and Jeff the Spider, who infects Mandy with "Spider Pox".
In Billy & Mandy Begins, Billy, Mandy and Grim recount the beginning of their friendship, including two fantastic stories and the real story from the first episode in the series.
At the beach, Billy wants to dump sand on Grim for fun. Mandy told him to go swim in the ocean. While he's having fun, a whale ate Billy. Inside, he finds Pinnochio and his papa, Geppeto. Grim and Mandy had to find the whale and get Billy out of there.
Billy wants to learn how to juggle chickens. He visits the chicken juggler legend, and the background Billy's in is based on retro black-and-white cartoons. Grim and Mandy finds Billy as a retro cartoon character while juggling chickens and having fun. After Mandy yelled that she hates retro cartoons, the phonograph turns the whole world into a black-and-white cartoon from the 1920s. How will they go back to their normally modern and colorful selves again?
Billy's dad has gotten a new job. Billy and his family are going to have to move to another part of town. However, he's always lived near to Mandy, and the two of them share the Grim Reaper. By Billy's relocating, this means that they have a problem. Who gets to "keep" Grim? Should Billy be the sole "owner" of Grim, and take him with him upon moving. Or should Grim be given to Mandy, and stay with her? To resolve this, both of them take their dispute to the Underworld Court, where a jury of hideous, nightmarish creatures (including one really annoying one) will decide once and for all who is truly the "Keeper of the Reaper."
When Billy's Mom asked what Billy wants to be when he grows up, he wants to be the grim reaper. So Grim showed Billy how to be the messenger of death. When Billy saw a girl that looks like him, he becomes smittened. But this girl is trouble when Blandy actually loves Grim. Billy becomes jealous and beats the crud out of Grim. What kind of monster Blandy is anyway?
Tired of having to deal with false alarms and giving out bonus life points, Grim visits Mighty Moe. Grim gave Mighty Moe an award for staying healthy and alive longer than anyone else. Grim finds out how to end Moe's life. So he dresses up like a snack that tastes like cheese. Will he ever get Mighty Moe to eat the cheese snack?
While Billy was trying to dig his way to the Earth's core, he stumbles upon a frozen caveman. It's Fred Flintstone! Grim and Billy thawed him out of the ice. Grim wants to reap Fred's soul, but Billy wants to keep him as a friend. At school, he presented the caveman to the class. Then Fred goes crazy and kidnaps Mrs. Butterbean.
Billy and Irwin go to mystical Japan to watch large monsters battle each other. Grim decides to take Billy and Irwin to see the monsters in action. He turns Billy into a giant warrior so that he can enjoy sparring with the giant turtle monster. Mandy comes to Japan to find Grim, Billy, and Irwin causing troubles.
In order to become cool, Billy and Irwin join a football team, so they won't get bullied anymore. Meanwhile, Mandy took over Mindy's position as the new cheerleader captain.
After getting his kite stuck in the tree, Billy, trying to take off the kite, throws many other things on the three, including Mandy. He asks Grim for a help, but Grim's powers are useless, as there is too much nature in the tree. They ask Basil the Druid for an assistance. He wants to help, but three druids are needed for the ritual, so Basil teaches Grim and Billy to the druid skills.
Skarr was doing a lot of gardening in order to win the "Good Neighbor Award". When he spotted a weed, Skarr tries to exterminate it. He called Grim for assistance. Then Billy comes and ruins Skarr's garden with the mower. Strange glowing tendrils wrap Skarr around and gave him the powers to bring plants to life.
While at Costmo, the gang finds out that everything is too orderly. After talking to Hoss Delgado about it, they discovered that's because he's dating Eris. There isn't that much chaos anymore. So it's up to them to "stop order from taking over."
When Grim gets a day off at the same time when Billy visits his grandma and Mandy does her home spa treatment, he spends his time dealing with huge lawn gnomes when Skarr takes his scythe. With both Billy and Mandy busy, he ends up getting help from Mindy and Pud'n.
When a girl named Dora comes to Billy and Mandy's school, she and Mandy become instant friends. While watching Mindy and the cheerleaders, Dora tells Mandy that if she opens her lunchbox. There's something that could destroy Mindy. Since Mandy wants to destroy Mindy, she opens the lunchbox, unleashing evil demons shaped like food. It turns out that Dora is actually Pandora, and she tricked Mandy into unleashing all of the demons. Mandy, angry that Dora tricked her, stops the end of the world by sucking the demons and Dora into the lunchbox.
When Billy destroys Grim's favorite childhood toy, Fancy Jake, Grim feels devastated and wishes he had never lost the limbo game that made him become Billy and Mandy's friend. Then, a lawyer appears saying that although the game is legally binding while all parties remain on Earth, it is not if Grim leaves Earth. In consequence, Grim follows the lawyer's advice, opens a portal, and leaves. Then, with Grim travelling aboard his spaceship, the lawyer takes off his disguise and introduces himself as Morg, the Martian Grim Reaper, who is also Emperor of Mars and commands an army of martian, brain-eating zombies. Then, so Grim can get the respect he deserves, Morg convinces Grim to conquer Earth, which they do easily while enslaving and turning all the people into zombies. Next for the reapers is to use the ultimate weapon in mind control, the Braincaster, which, if powered by a dumb mind, will allow them to control all the zombies forever. Meanwhile, Billy, Mandy and Irwin, who followed Grim to Mars hidden in his bottomless trunk, look for a way to save Earth. Then, when the kids are captured, and Billy is strapped to the Braincaster, all hope seems lost; however, at the last minute Billy talks to Grim about friendship, and Grim has a change of heart and decides to help Mandy. In the end, Billy's stupid brain overpowers the Braincaster, returning all the people to normal and foiling Morg's plan, who disappears into outer space.
As Billy sees Hoss Delgado's manliness is action again, Billy wishes to grow up to be like him, in other words, to be a hero. Then, Grim tells Billy that in older times an evident way to become a hero was to slay a dragon, so they travel to the Middle Ages so Billy can train under the supervision of the best dragon slayer, Sir Boss Del Guapo, who resembles the present time Delgado. In the end, Billy becomes friends with the dragon he was supposed to kill and spares his life, becoming the dragon's hero. Also, Del Guapo finds himself transported to the present, where he has brief encounter with Delgado.
When Grim decides to visit again his childhood idol, the aged king of the vampires, Dracula, the latter suggest they drive to an early bird buffet, only to get lost in the scorching desert. Then, Grim and Dracula need to find their way back home while dealing with situations like meeting a creepy truck driver and being attacked by a giant scorpion. After what they have gone through, and thinking that Dracula is just a selfish, old man, Grim decides to go a separate way; however, after finding that Dracula in fact helped him by sucking out the scorpion's venom, Grim rushes to help Dracula, and they finally make it to the diner. However, in the end, it appears that Dracula left without paying the check.
As Billy and Mandy celebrate Father's Day, Grim becomes very depressed that he sits inside Billy's closet only listening to country rock music. Then, Grim explains that, since his own father cared more about country rock than about him, he has been living a lie, telling his father he is a famous country rock singer, and not the actual Grim Reaper. Then, Mandy arranges for Grim's dad to come visit and she tells him the truth, but still the old man doesn't believe her. Grim decides that the only way for his dad to listen to him is while performing country music. Then, at an outdoor concert, Billy, Mandy and Grim start performing a country rock song, in which eventually Grim reveals his supernatural powers. In the end, Grim's dad accepts his son is the Reaper, but still doesn't forgive him.
As Mandy returns from Billy's house, it is evident that Mandy parents are extremely afraid of their daughter and her iron fist rule over them. Unable to live in fear any longer, Mandy's parents call a nanny from a reality TV show who specializes in disciplining spoiled children. Then, Mandy and the nanny clash several times, while the nanny tries to teach Mandy's parents to show authority to control their daughter. In the end, Grim helps the nanny in her feud against Mandy by using his scythe to make Mandy's parents more resolute; however, Mandy quickly restores her control over her parents, and shows the nanny she actually controls everything and everyone.
When Mandy says even a trained monkey is more competent to do cleaning chores than Grim, and would complain less, Grim makes Dickie Galoot, a monkey appearing in a Japanese-styled animation, come to life to prove her wrong. Then, as Dickie turns out to be extremely good at cleaning Mandy's house, Grim decides to leave the kids for good; however, due to his friendship contract, Grim is unable to break free, and so he is forced to battle Dickie to recover his place as the "cleaning monkey" of the group. In the end, Grim successfully defeats Dickie at cleaning Billy's house, but this all turns out to be an ingenious plan devised by Mandy to win a bet against Billy.
At a school fair, Grim's scythe accidentally transforms the dull Principal Goodvibes and his hippopotamus into gangster rappers, Goodvibes now calling himself Principal Goodbling. In the following, Goodbling changes the entire school to be more hip-hop oriented, to the annoyance of Mandy. Although initially Grim likes the new style of the school principal, he later thinks Goodbling is just a poseur and goes to talk to him, but Goodbling attacks with yo-mama jokes. In the end, Irwin's grandmother, Tanya, confronts Goodbling and proves to be better than he is at yo-mama jokes that Goodbling reverts to being the boring Goodvibes.
When Billy finds his "son" Jeff the Spider lying in his bed with "Spider Pox", he goes to Mandy for help to get rid of the giant spider. While ignoring Billy, the next morning Mandy awakens afflicted by Spider Pox too, developing four additional arms and the ability to produce spider web. Later, Mandy's condition progresses and she completely turns into a giant spider, who then captures Billy's parents and Grim. Then, Grim tells Billy that, to cure Mandy, he must use the venom of the spider that infected her. In the end, while Billy successfully gives Mandy Jeff's venom, he also carried the virus and infected the people of the city, who all turned into spiders.
As Grim appears in an Underworld TV commercial for "Big Move" laxative pills, an accompanying contest promises a day with the famous Grim Reaper to the monster who writes the best 500-word-or-less essay describing the fun things they would do. It turns out the winner of the contest, and sole participant, is the dim-witted Fred Fredburger, who only wants to eat frozen yoghurt. Also, the Big Move company blames Grim for their sales going down since they hired him. In consequence, Grim tries save his reputation and make the best day for Fred, but it is a disaster with Grim ruining Fred's yoghurt and Fred going missing in an amusement park. In the end, the Big Move executives give Grim a beating for ruining their company, and Fred finds his own paradise in a cold place with nachos- and frozen yoghurt-loving creatures.
The Crass Unicorn: There's trouble in the Enchanted Forest, and Billy finds a unicorn who is a real jerk. Billy & Mandy Begins: Billy, Mandy, and Grim each narrate a different version about how they met.
As Billy, Mandy, Grim, and Irwin hang from a rope about to be eaten by a giant sea serpent, Irwin wonders how they came into that trouble. Then, Billy and Grim each tell disparate stories on how they became best friends originally. Billy's fantastic story is about he and Mandy being space patrol officers who collect the Grim Reaper's bones, cloak, and skull to summon him; Grim tells a story on how the evil Mandy and his pet servant Billy managed to capture him inside a containment unit (parodying the Ghostbusters' ghost traps). In the end, Mandy quickly tells the real story, and footage from the first episode is seen, before the four are swallowed whole by the serpent.